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FreeBSD ZFS RAIDZ failed disk replacement

(for the rest of us)

I don’t know about you but most of my ZFS arrays are large, using SAS drives connected via SAS HBAs

to expanders that know which disk is where. I also have multiple redundancy in the zpool and hot spares, so I don’t need pay a visit just to replace a failed disk. And if I do I can get the enclosure to flash an LED over the drive I’m interested in replacing.

Except at home. At home I’ve got what a lot of people probably have. A small box with a four-drive cage running RAIDZ1 (3+1). And it’s SATA, because it really is a redundant array of independent drives. I do, of course, keep a cold spare I can swap out. Always make sure you you have at least one spare drive of the dimensions of a RAIDZ group, and know where you find it.

And to make it even more fun, you’re booting from the array itself.

After many years I started getting an intermittent CAM error, which isn’t good news. Either one of the drives was loose, or it was failing. And there’s no SAS infrastructure to help. If you’re in a similar position you’ve come to the right place.

WARNING. The examples in this article assume ada1 is the drive that’s failed. Don’t blindly copy/paste into a live system without changing this as appropriate

To change a failed or failing drive:

  • Find the drive
  • Remove the old drive
  • Configure the new drive
  • Tell the RAIDZ to use it

Finding the failed drive

First, identify your failing drive. The console message will probably tell you which one. ZFS won’t, unless it’s failed to the extent it’s been offlined. “zpool status” may tell you everything’s okay, but the console may be telling you:

Feb  2 17:56:23 zfs1 kernel: (ada1:ahcich1:0:0:0): CAM status: ATA Status Error
Feb  2 17:56:23 zfs1 kernel: (ada1:ahcich1:0:0:0): CAM status: ATA Status Error
 Feb  2 17:56:23 zfs1 kernel: (ada1:ahcich1:0:0:0): ATA status: 41 (DRDY ERR), error: 40 (UNC )
 Feb  2 17:56:23 zfs1 kernel: (ada1:ahcich1:0:0:0): RES: 41 40 b0 71 20 00 f6 00 00 00 01
 Feb  2 17:56:23 zfs1 kernel: (ada1:ahcich1:0:0:0): Retrying command, 0 more tries remain
 Feb  2 17:56:23 zfs1 kernel: (ada1:ahcich1:0:0:0): READ_FPDMA_QUEUED. ACB: 60 00 d8 70 20 40 f6 00 00 01 00 00
 Feb  2 17:56:23 zfs1 kernel: (ada1:ahcich1:0:0:0): CAM status: ATA Status Error
 Feb  2 17:56:23 zfs1 kernel: (ada1:ahcich1:0:0:0): ATA status: 41 (DRDY ERR), error: 40 (UNC )
 Feb  2 17:56:23 zfs1 kernel: (ada1:ahcich1:0:0:0): RES: 41 40 b0 71 20 00 f6 00 00 00 01
 Feb  2 17:56:23 zfs1 kernel: (ada1:ahcich1:0:0:0): Error 5, Retries exhausted

So this tells you that ada1 is in a bad way. But which is ada1? It might be the second one across in your enclosure, or it might not. You’ll need to do some hunting to identify it positively.

Luckily most disks have their serial number printed on the label, and this is available to the host. So finding the serial number for ada1 and matching it to the disk label is the best way – if you’ve only got four drives to check, anyway.

I know of at least five ways to get a disk serial number in FreeBSD, and I’ll list them all in case one stops working:

dmesg

Just grep for the drive name you’re interested in (ada1). This is probably a good idea as it may give you more information about the failure. If FreeBSD can get the serial number it will display it as it enumerates the drive.

geom disk list

This will print out information on all the geoms (i.e. drives), including the serial number as “ident”

camcontrol identify ada1

This gives you more information that you ever wanted about a particular drive. This does include the serial number.

diskinfo -s /dev/ada1

This simply prints the ident for the drive in question. You can specify multiple arguments so diskinfo -s /dev/ada? works (up to ten drives).

smartctl -i /dev/ada1

Smartctl is a utility for managing SATA drives (not SAS!), and you should probably install it. It’s part of Smartmontools as it gives you the ATA information for drive, including errors rates, current temperature and suchlike.

Whichever method works for you, once you’ve got your serial number you can identify the drive. Except, of course, if your drive is completely fubared. In that case get the serial numbers of the drives that aren’t and identify it by elimination.

Saving the partition table from the failed drive.

In readiness for replacing it, if you can save its partition table:

gpart backup ada1 >
gpart.ada1

If you can’t read it, just save another one from a different drive in the vdev set – they should be identical, right?

Swapping the bad drive out.

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Next, pull your faulty drive and replace it with a new one. You might want to turn the power off, although it’s not necessary. However, it’s probably safer to reboot as we’re messing with the boot array.

Try zpool status, and you’ll see something like this:

  pool: zrpool: zr
  state: DEGRADED
 status: One or more devices could not be opened. 
         Sufficient replicas exist for
         the pool to continue functioning in a degraded state.
 action: Attach the missing device and online it using 'zpool online'.
   scan: scrub in progress since Sun Feb  2 17:42:38 2025
         356G scanned at 219/s, 175G issued at 108/s, 10.4T total
         0 repaired, 1.65% done, no estimated completion time
 config:
     NAME                   STATE     READ WRITE CKSUM
     zr                     DEGRADED     0     0     0
       raidz1-0             DEGRADED     0     0     0
         ada0p3             ONLINE       0     0     0 
         16639665213947936  UNAVAIL      0     0     0  was /dev/ada1p3
         ada2p3             ONLINE       0     0     0
         ada3p3             ONLINE       0     0     0

It’s complaining because it can’t find the drive with the identity 16639665213947936. ZFS doesn’t care where the drives in a vdev are plugged in, only that they exist somewhere. Device ada1 is ignored – it’s just got some random disk in that zfs isn’t interested in.

Setting up the replacement drive

So let’s get things ready to insert the new drive in the RAIDZ.

First restore its partition table:

gpart restore
/dev/ada1 < gpart.ada1

If you see “gpart: geom ‘ada1’: File exists”, just run “gpart destroy -F ada1”. Without the -F it may say the drive is in use, which we know it isn’t.

Next, if you’ve got a scrub going on, stop it with “zpool scrub -s zr”

As a sanity check, run “gpart show” and you should see four identical drives.

Boot sector and insertion

Now this is a boot from ZFS situation, common on a home server but not a big one. The guides from Solaris won’t tell you about this step. To make sure the system boots you need to have the boot sector on every drive (ideally). Do this with:

gpart bootcode -b /boot/pmbr -p /boot/gptzfsboot -i 1 ada1

And Finally… tell ZFS to insert the new drive:

 zpool replace ada1p3

Run “zpool status” and you’ll see it working:

pool: zr
  state: DEGRADED
 status: One or more devices is currently being resilvered.  The pool will
         continue to function, possibly in a degraded state.
 action: Wait for the resilver to complete.
   scan: resilver in progress since Sun Feb  2 18:44:58 2025
         1.83T scanned at 429M/s, 1.60T issued at 375M/s, 10.4T total
         392G resilvered, 15.45% done, 0 days 06:48:34 to go
 config:
     NAME                     STATE     READ WRITE CKSUM
     zr                       DEGRADED     0     0     0
       raidz1-0               DEGRADED     0     0     0
         ada0p3               ONLINE       0     0     0
         replacing-1          UNAVAIL      0     0     0
           16639665213947936  UNAVAIL      0     0     0  was /dev/ada1p3/old
           ada1p3             ONLINE       0     0     0
         ada2p3               ONLINE       0     0     0
         ada3p3               ONLINE       0     0     0
 errors: No known data errors


It’ll chug along in the background re-silvering the whole thing. You can carry on using the system, but it’s performance may be degraded until it’s done. Take a look at the console to make sure there are no CAM errors indicating that the problem wasn’t the drive at all, and go to bed.
If you reboot or have a power cut while its rebuilding it will start from scratch, so try to avoid both!

In the morning, zpool status will return to this, and all will be well in the world. But don’t forget to order another cold spare so you’re ready when it happens again.

pool: zr
  state: ONLINE
   scan: resilvered 2.47T in 0 days 11:52:45 with 0 errors on Mon Feb  3 06:37:43 2025
 config:
     NAME        STATE     READ WRITE CKSUM
     zr          ONLINE       0     0     0
       raidz1-0  ONLINE       0     0     0
         ada0p3  ONLINE       0     0     0
         ada1p3  ONLINE       0     0     0
         ada2p3  ONLINE       0     0     0
         ada3p3  ONLINE       0     0     0
 errors: No known data errors

As a final tip, if you use diskinfo -v adaX it will tell you the type of drive and other information, which is really handy if you’re ordering another cold spare.

# diskinfo -v adaa
 ada2
         512             # sectorsize
         3000592982016   # mediasize in bytes (2.7T)
         5860533168      # mediasize in sectors
         4096            # stripesize
         0               # stripeoffset
         5814021         # Cylinders according to firmware.
         16              # Heads according to firmware.
         63              # Sectors according to firmware.
         HGST HUS724030ALE640    # Disk descr. <- Drive to order on eBay!
         PK2234P9GWJ42Y  # Disk ident.
         No              # TRIM/UNMAP support
         7200            # Rotation rate in RPM
         Not_Zoned       # Zone Mode


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How hot is your FreeBSD machine?

FreeBSD may be the hottest operating system available, but having hot hardware isn’t so good. Modern drives and CPUs can report their temperature, but it’s not easy to see it.

I’ve produced an example script that will report the temperature of whatever it can, with the idea that it can form the basis of whatever you really need to do. I have a multi-server monitoring system using the same methods.

Getting the CPU core temperature is a simple sysctl variable, but it only appears if you have the coretemp.ko module loaded or compiled into the kernel. But coretemp.ko only works for Intel processors; for AMD you need amdtemp.ko instead.

The script tries to determine the CPU type the best way I know how, and loads the appropriate module if necessary. If it loads the module, it unloads the module at the end to leave things as it found them. You can omit the unload so it’s loaded on first, or put the module permanently in loader.conf if you prefer. But this is only an example, and I prefer to keep my scripts independent of host configuration.

Next you need a way to get the temperature from all your drives. This isn’t built in to FreeBSD but you can use the excellent the excellent smartmontools https://www.smartmontools.org by Bruce Allen and Christian Franke.

This was originally intended to access the SMART reporting on ATA drives, but will now extract information for SAS units too. To smartmontools, and the “smartctl” utility in particular, you can build from ports with:

cd /usr/ports/sysutils/smartmontools
make install clean

You can also install it as a binary package with: “pkg install smartmontools”

The script tries to enumerate the drives and CPUs on the system. ATA drives are in /dev/ and begin “ada”, SCSI and USB drives begin “da”. The trick is to figure out which devices are drives and which are partitions or slices within a drive – I don’t have a perfect method.

SCSI drives that aren’t disks (i.e. tape) start “sa”, and return some really weird stuff when smartctl queries them. I’ve tested the script with standard LTO tape drives, but you’ll probably need to tweak it for other things. (Do let me know).

Figuring out the CPUs is tricky, as discrete CPUs and cores within a single chip appear the same. The script simply goes on the cores, which results in the same temperature being reported for each.

You can override any of the enumeration by simply assigning the appropriate devices to a list, but where’s the fun? Seriously, this example shows how you can enumerate devices and it’s useful when you’re monitoring disparate hosts using the same script.

Finally, there are three loops that read the temperature for each device type into into “temp” and then print it. Do whatever you want – call “shutdown -p now” if you think something’s too hot; autodial the fire brigade or, as I do, send yourself an email.

The Script 

#!/bin/sh
# FreeBSD Temperature monitoring example script 
# (c) FJL 2024 frank@fjl.co.uk
# Please feel free to use this as an example for a few techniques 
# including enumerating devices on a host and extracting temperature 
# information.

# Full path to utilities in case run with no PATH set 
GREP=/usr/bin/grep SMARTCTL=/usr/local/sbin/smartctl 
CUT=/usr/bin/cut 
SYSCTL=/sbin/sysctl 

# Load the AMD CPU monitoring driver if necessary 
if [ ! $($SYSCTL -n dev.cpu.0.temperature 2>/dev/null) ] 
then
    # Let's try to find out if we have Intel or
    # AMD processor and select correct kernel module         
    if $SYSCTL -n hw.model | $GREP AMD >/dev/null
         then
                 tempmodule=amdtemp
         else
                 tempmodule=coretemp
         fi
         # Load the CPU temp kernel module
         kldload $tempmodule
         # Set command to unload it when we're done (optional)
         unload="kldunload $tempmodule"
    fi 
# Enumerate SATA, USB and SAS disks - everything
# in /dev/ starting da or ada, except where there's an s or p in it
disks=$(find /dev -depth 1 \( -name da\* -o -name ada\* \) -and -not -name \*s\* -and -not -name \*p\* | cut -c 6- | sort ) 

# Enumerate other SCSI devices, starting in sa.
# Normally tape drives. May need tweaking!
scsis=$(find /dev -depth 1 \( -name sa\* \) -and -not -name \*.ctl | cut -c 6- | sort)

# Enumerate the CPUs
cpus="$(seq 0 $(expr $($SYSCTL -n hw.ncpu ) - 1))"

# Print all the ATA disks
for disk in $disks
do
         temp=$($SMARTCTL -a /dev/$disk | $GREP Temperature_Celsius | $CUT -w -f 10)
         echo "$disk: ${temp}C" done

# Print all the SCSI devices (e.g. tapes)
# NB. This will probably take a lot of fiddling as SCSI units return all sorts to smartctl
# Note the -T verypermissive. see man smartctl for details.
for scsi in $scsis
do
         temp=$($SMARTCTL -a -T verypermissive /dev/$scsi | $GREP "Current Drive Temperature" | cut -w -f 4)
         echo "$scsi: ${temp}C"
done

# Print all the CPUs
for cpu in $cpus
do
         temp=$($SYSCTL -n dev.cpu.$cpu.temperature | $CUT -f 1 -d .)
         echo "CPU$cpu: ${temp}C"
done

# Unload the CPU temp kernel module if we loaded it (optional)
$unload

Example Output

ada0: 35C
ada1: 39C
ada2: 39C
ada3: 38C
da0: 24C
da1: 25C
sa0: 33C
CPU0: 48C
CPU1: 48C

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Why people obsess about the ZFS SLOG, but shouldn’t

There are two mysteries things on ZFS that cause a lot of confusion: The ZIL and the SLOG. This article is about what they are and why you should care, or not care about them. But I’ll come to them later. Instead I’ll start with POSIX, and what it says about writing stuff to disk files.

When you write to disk it can either be synchronous or asynchronous. POSIX (Portable Operating System Interface) has requirements for writes through various system calls and specifications.

With an asynchronous write the OS takes the data you give it and returns control to the application immediately, promising to write the data as soon as possible in the background. No delay. With a synchronous write the application won’t get control back until the data is actually written to the disk (or non-volatile storage of some kind). More or less. Actually, POSIX.1-2017 (IEEE Standard 1003.1-2017) doesn’t guarantee it’s written, but that’s the expectation.

You’d want synchronous writes for critical complex files, such as a database, where the internal structure would break if a transaction was only half written, and a database engine needs to know that one write has occurred before making another.

Writes to ZFS can be long and complicated, requiring multiple blocks be updated for a single change. This is how it maintains its very high integrity. However, this means it can take a while to write even the simplest thing, and a synchronous write could take ages (in computer terms).

To get around this, ZFS maintains a ZIL – ZFS Intent Log.

In ZFS, the ZIL primarily serves to ensure the consistency and durability of write operations, particularly for synchronous writes. But it’s not a physical thing; it’s a concept or list. It contains transaction groups that need to be completed in order.

The ZIL can be physically stored in three possible places…

In-Memory (Volatile Storage):

This is the default location. Initially, all write operations are buffered in RAM. This is where they are held before being committed to persistent storage. This kind ofZIL is volatile because it’s not backed by any permanent storage until written to disk.

Volatility doesn’t matter, because ZFS guarantees consistency with transaction groups (TXGs). The power goes off and the in-RAM ZIL is lost, the transactions are never applied; but the file system is in a consistent state.

In-Pool (Persistent Storage):

Without a dedicated log device, the ZIL entries are written to the main storage pool in transaction groups . This happens for both synchronous and asynchronous writes but is more critical for synchronous writes to ensure data integrity in case of system crashes or power failures.

SLOG (Separate Intent Log Device):

For better performance with synchronous writes, you can add a dedicated device to serve as the SLOG. This device is typically a low-latency, high-speed storage like a short-stroked Rapter, enterprise SSD or NVRAM. ZFS writes the log entries before they’re committed to the pool’s main storage.

By storing the pending TXGs on disk, either in the pool or on an SLOG, ZFS can meet the POSIX requirement that the transaction is stored in non-volatile storage before the write returns, and if you’re doing a lot of synchronous writes then storing them on a high-speed SLOG device helps. But only if the SLOG device is substantially faster than an array of standard drives. And it only matters if you do a lot of synchronous writes. Caching asynchronous writes in RAM is always going to be faster still

I’d contend that the only times synchronous writes feature heavily are databases and virtual machine disks. And then there’s NFS, which absolutely loves them. See ESXi NFS ZFS and vfs-nfsd-async for more information if this is your problem.

If you still think yo need an SLOG, install a very fast drive. These days an NVMe SLC NAND device makes sense. Pricy, but it doesn’t need to be very large. You can add it to a zpool with:

zpool add poolname
log /dev/daX

Where daX is the drive name, obviously.

As I mentioned, the SLOG doesn’t need to be large at all. It only has to cope with five seconds of writes, as that’s the maximum amount of time data is “allowed” to reside there. If you’re using NFS over 10Gbit Ethernet the throughput isn’t going to be above 1.25Gb a seconds. Assuming that’s flat-out synchronous writes, multiplying that by five seconds is less than 8Gb. Any more would be unused.

If you’ve got a really critical system you can add mirrored SLOG drives to a pool thus:

zpool add poolname
log /dev/daX /dev/daY

You can also remove them with something like:

zpool remove
poolname log /dev/daY

This may be useful if adding an SLOG doesn’t give you the performance boost you were hoping for. It’s very niche!

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mysql/mariadb error 35

I’ve had a problem with mysql failing to start with error 35 in the log file:

InnoDB: Unable to lock ./ibdata1, error: 35
InnoDB: Check that you do not already have another mysqld process
InnoDB: using the same InnoDB data or log files.

What to do? Google and you get a lot of Linux people saying that the answer is to reboot the box. Hmm. Well you don’t have to.

What causes the error is mysqld crashing, usually when system resources are exhausted. Rebooting will, indeed, unlock ibdata1 but so will killing the process that locks it. Yet the server isn’t running, so how can this be? Well actually part of it is – just not the part the service manager sees.

Run “ps -auxww | grep mysql” and you’ll find a few more. Send them a kill, wait for it to work and then restart. Obviously you can only do this and expect it to work if you’ve sorted out the resource problem.

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FreeBSD 14 ZFS warning

Update 27-Nov-23
Additional information has appeared on the FreeBSD mailing list:
https://lists.freebsd.org/archives/freebsd-stable/2023-November/001726.html

The problem can be reproduced regardless of the block cloning settings, and on FreeBSD 13 as well as 14. It’s possible block cloning simply increased the likelihood of hitting it. There’s no word yet about FreeBSD 12, but this FreeBSD’s own ZFS implementation so there’s a chance it’s good

In the post by Ed Maste, a suggested partial workaround is to set the tunable vfs.zfs.dmu_offset_next_sync to zero, which has been on the forums since Saturday. This is a result of this bug:

https://bugs.freebsd.org/bugzilla/show_bug.cgi?id=275308

There’s a discussion of the issue going on here:

https://forums.freebsd.org/threads/freebsd-sysctl-vfs-zfs-dmu_offset_next_sync-and-openzfs-zfs-issue-15526-errata-notice-freebsd-bug-275308.91136/

I can’t say I’m convinced about any of this.

–
FreeBSD 14, which was released a couple of days ago, includes OpenZFS 2.2. There’s a lot of suspicion amongst Gentoo Linux users that this has a rather nasty bug in it related to block cloning.

Although this feature is disabled by default, people might be tempted to turn it on. Don’t. Apparently it can lead to lost data.

OpenZFS 2.2.0 was only promoted to stable on 13th October, and in hindsight adding it to a FreeBSD release so soon may seem precipitous. Although there’s a 2.2.1 release you should now be using instead it simply disables it by default rather than fixing the likely bug (and to reiterate, the default is off on FreeBSD 14).

Earlier releases of OpenZFS (2.1.x or earlier) are unaffected as they don’t support block cloning anyway.

Personally I’ll be steering clear of 2.2 until this has been properly resolved. I haven’t seen conclusive proof as to what’s causing the corruption, although it looks highly suspect. Neither have I seen or heard of it affecting the FreeBSD implementation, but it’s not worth the risk.

Having got the warning out of the way, you may be wondering what block cloning is. Firstly, it’s not dataset cloning. That’s been working fine for years, and for some applications it’s just what’s needed.

Block cloning applies to files, not datasets, and it’s pretty neat – or will be. Basically, when you copy a file ZFS doesn’t actually copy the data blocks – it just creates a new file in the directory structure but it points to the existing blocks. They’re shared between the source and destination files. Each block has a reference count in the on-disk Block Reference Table (BRT), and only when a block in the new file changes does a copy-on-write occur; the new block is linked to the new file and the reference count in the BRT is decremented. In familiar Unix fashion, when the reference count for a block gets to zero it joins the free pool.

This isn’t completely automatic – it must be allowed when the copy is made. For example, the cp utility will request clone files by default. This is done using the copy_file_range() system call with the appropriate runes; simply copying a file with open(), read(), write() and close() won’t be affected.

As of BSDCAN 2023, there was talk about making it work with zvols but this was to come later, although clone blocks in files can exist between datasets as long as they’re using the same encryption (including keys).

One tricky problem here is how it works with the ZIL – for example what’s stopping a block pointer from disappearing from the log? There was a lot to go wrong, and it looks like it has.

Release notes for 2.2.1 may be found here.
https://github.com/openzfs/zfs/releases/tag/zfs-2.2.1

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Using ddrescue to recover data from a USB flash drive

If you’re in the data recovery, forensics or just storage maintenance business (including as an amateur) you probably already know about ddrescue. Released about twenty years ago by Antonio Diaz Diaz, it was a big improvement over the original concept dd_rescue from Kurt Garloff in 1999. They copy disk images (which are just files in Unix) trying to get as much data extracted when the drive itself has faults.

If you’re using Windows rather than Unix/Linux then you probably want to get someone else to recover your data. This article assumes FreeBSD.

The advantage of using either of these over dd or cp is that they expect to find bad blocks in a device and can retry or skip over them. File copy utilities like dd ignore errors and continue, and cp will just stop. ddrescue is particularly good at retrying failed blocks, and reducing the block size to recover every last readable scrap – and it treats mechanical drives that are on their last legs as gently as possible.

If you’re new to it, the manual for ddrescue can be found here. https://www.gnu.org/software/ddrescue/manual/ddrescue_manual.html

However, for most use cases the command is simple. Assuming the device you want to copy is /dev/da1 and you’re calling it thumbdrive the command would be:

ddrescue /dev/da1
thumbdrive.img thumbdrive.map

The device data would be stored in thumbdrive.img, with ongoing state information stored in thumbdrive.map. This state information is important, as it allows ddrescue to pick up where it left off.

However, ddrescue was written before USB flash drives (pen drives, thumb drives or whatever). That’s not to say it doesn’t work, but they have a few foibles of their own. It’s still good enough that I haven’t modified ddrescue base code to cope, but by using a bit of a shell script to do the necessary.

USB flash drives seem to fail in a different way to Winchester disks. If a block of Flash EPROM can’t be read it’s going to produce a read error – fair enough. But they have complex management software running on them that attempts to make Flash EPROM look like a disk drive, and this isn’t always that great in failure mode. In fact I’ve found plenty of examples where they come across a fault and crash rather than returning an error, meaning you have to turn them off and on to get anything going again (i.e. unplug them and put them back in).

So it doesn’t matter how clever ddrescue is – if it hits a bad block and the USB drive controller crashes the it’s going to be waiting forever for a response and you’ll just have come reset everything manually and resume. One of the great features of ddrescue is that it can be stopped and restarted at any time, so continuing after this happens is “built in”.

In reality you’re going to end up unplugging your USB flash drive many times during recovery. But fortunately, it is possible to turn a USB device off and on again without unplugging it using software. Most USB hardware has software control over its power output, and it’s particularly easy on operating systems like FreeBSD to do this from within a shell script. But first you have to figure out what’s where in the device map – specifically which device represents your USB drive in /dev and which USB device it is on the system. Unfortunately I can’t find a way of determining it automatically, even on FreeBSD. Here’s how you do it manually; if you’re using a version of Linux it’ll be similar.

When you plug a USB storage device into the system it will appear as /dev/da0 for the first one; /dev/da1 for the second and so on. You can read/write to this device like a file. Normally you’d mount it so you can read the files stored on it, but for data recovery this isn’t necessary.

So how do you know which /dev/da## is your media? This easy way to tell is that it’ll appear on the console when you first plug it in. If you don’t have access to the console it’ll be in /var/log/messages. You’ll see something like this.

Jun 10 17:54:24 datarec kernel: umass0 on uhub5
kernel: umass0: <vendor 0x13fe USB DISK 3.0, class 0/0, rev 2.10/1.00, addr 2> on usbus1
kernel:  umass0 on uhub5
kernel:  umass0:  on usbus1
kernel:  umass0:  SCSI over Bulk-Only; quirks = 0x8100
kernel:  umass0:7:0: Attached to scbus7
kernel:  da0 at umass-sim0 bus 0 scbus7 target 0 lun 0
< USB DISK 3.0 PMAP> Removable Direct Access SPC-4 SCSI device
kernel:  da0: Serial Number 070B7126D1170F34
kernel:  da0: 40.000MB/s transfers
kernel:  da0: 59088MB (121012224 512 byte sectors)
kernel: da0: quirks=0x3
kernel: da0: Write Protected

So this is telling us that it’s da0 (i.e /dev/da0)

The hardware identification is “<vendor 0x13fe USB DISK 3.0, class 0/0, rev 2.10/1.00, addr 2> on usbus1” which means it’s on USB bus 1, address 2.

You can confirm this using the usbconfig utility with no arguments:

ugen5.1:  at usbus5, cfg=0 md=HOST spd=HIGH (480Mbps) pwr=SAVE (0mA)
 ...snip...
ugen1.1:  at usbus1, cfg=0 md=HOST spd=HIGH (480Mbps) pwr=SAVE (0mA)
ugen1.2:  at usbus1, cfg=0 md=HOST spd=HIGH (480Mbps) pwr=ON (300mA)

There it is again, last line.

usbconfig has lots of useful commands, but the ones we’re interested are power_off and power_on. No prizes for guessing what they do. However, unless you specify a target then it’ll switch off every USB device on the system – including your keyboard, probably.

There are two ways of specifying the target, but I’m using the -d method. We’re after device 1.2 so the target is -d 1.2

Try it and make sure you can turn your USB device off and on again. You’ll have to wait for it to come back online, of course.

There are ways of doing this on Linux by installing extra utilities such as hub-ctrl. You may also be able to do it by writing stuff to /sys/bus/usb/devices/usb#/power/level” – see the manual that came with your favourite Linux distro.

The next thing we need to do is provide an option for ddrescue so that it actually times out if the memory stick crashes. The default is to wait forever. The –timeout=25 or -T 25 option (depending on your optional taste) sees to that, making it exit if it hasn’t been able to read anything for 25 seconds. This isn’t entirely what we’re after, as a failed read would also indicate that the drive hadn’t crashed. Unfortunately there’s no such tweak for ddrescue, but failed reads tend to be quick so you’d expect a good read within a reasonable time anyway.

So as an example of putting it all into action, here’s a script for recovering a memory stick called duracell (because it’s made by Duracell) on USB bus 1 address 2.

#!/bin/sh
while ! ddrescue -T 25 -u /dev/da0 duracell.img duracell.map
 do
     echo ddrescue returned $?
     usbconfig -d 1.2 power_off
     sleep 5
     usbconfig -d 1.2 power_on
     sleep 15
     echo Restarting
 done

A few notes on the above. Firstly, ddrescue’s return code isn’t defined. However, it appears to do what one might expect so the above loop will drop out if it ever completes. I’ve set the timeout for time since last good read to 25 seconds, which seems about right. Turning off the power for 5 seconds and then waiting for 15 seconds for the system to recognise it may be a bit long – tune as required. I’m also using the -u option to tell ddrescue to only go forward through the drive as it’s easier to read the status when it’s always incrementing. Going backwards and forwards makes sense with mechanical drives, but not flash memory.

Aficionados of ddrescue might want to consider disabling scraping and/or trimming (probably trimming) but I’ve seen it recover data with both enabled. Data recovery is an art, so tweak away as you see fit – I wanted to keep this example simple.

Now this system isn’t prefect. I’m repurposing ddrescue, which does a fine job on mechanical drives, to recover data from a very different animal. I may well write a special version for USB Flash drives but this method does actually work quite well. Let me know how you get on.

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Quicken doesn’t like Amex QIF format

If you’re trying to import data into Quicken 98 (or compatible) having downloaded it from the American Express web site you’ll get some odd and undesirable results. There are two incompatibilities.

Firstly the date field (D) needs to have a two-digit year, whereas Amex adds the century.

Secondly, for reasons I haven’t quite figured out yet, the extra information M field sometimes results is a blank entry (other than the date).

You can fix this by editing the QIF file using an editor of your choice, but this gets tedious so here’s a simple program that will do it for you. It reads from stdin and writes to stdout so you’ll probably use it in a script or redirect it from and to a file. I compile it to “amexqif”. If there’s any interest I’ll tweak it to make it more friendly.

It’s hardly a complex program, the trick was figuring out what’s wrong in the first place.

#include <stdio.h>
#include <string.h>

char buf [200];

int main()
{
int lenread;
char *str;
        while ((str = fgets(buf, 200,stdin))) // (()) to placate idiot mode on CLANG
        {
                if ((lenread = strlen(str)) > 1)        // Something other than just the newline
                {
                        if (str[0] == 'M')
                                continue;       // Random stuff in M fields breaks Q98


                        if (str[0] == 'D' && lenread == 12)     // Years in dates must be two digit
                                strcpy (str+7, str+9);
                }
                printf("%s",str);
        }
}
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Having a good argument

I’ve seen all sorts of stuff on forums about how to process command line argument in C or C++. What a load of fuss and bother. There’s a standard getopt() function in the ‘C’ library, similar to the shell programming command, but it’s not great.

The main problem with getopt() is that it produces its own error message. Although this saves you the trouble, it can be a bit user unfriendly for the user, especially when things get complex. For example, you might have mutually exclusive arguments and want to print out a suitable message. That said, it’ll work most of the time.

Here’s a page showing a good explanation for the GCC version, whcih is pretty standard.

Example of Getopt (The GNU C Library)

But rolling your own is not hard. Here’s a skeleton I use. It’s pretty self-explanatory. My rules allow single options (e.g. -a -b) or combined options (e.g. -ab), or any mixture. “–” ends options, meaning subsequent arguments are going to be a actual arguments.

If you want to pass something as an option, such as a filename, you can. -fmyfile or -f myfile are both handled in the example.

You can add code to detect a long option by adding “if (!strcmp(p,”longname”)) … just after the char c. But I don’t like long options.

#include <stdio.h>

void process(char *s)
{
    printf("Processing %s\n",s);
}

int main (int cnt, char **a)
{
    int i;
    for (i=1; i<cnt && a[i][0] == '-'; i++)
    {
        char *p = &a[i][1];
        char c;
        if (*p == '-')
        {
            i++;
            break;
        }
        while (c = *p)
            switch (*p++)
            {
            case 'a':
                printf("Option a\n");
                break;
                
            case 'b':
                printf("Option b\n");
                break;

            case 'f':
                if (!*p && i+1 < cnt)
                    printf("Value for f=%s\n", a[++i]);
                else
                {
                    printf("Value for f=%s\n", p);
                    while (*p)
                        p++;
                }
                break;
                
            default:
                printf("Bad switch %c\n",c);

            }
    }
    for (;i<cnt;i++)
        process(a[i]);
}

The above code assumes that options precede arguments. If you want to mix them the following code allows for complete anarchy – but you can end it using the “–” option, which will take any following flags as arguments. As a bonus it shows how to add a long argument.

#include <stdio.h>
#include <string.h>

void process(char *s)
{
    printf("Processing %s\n",s);
}

int main (int cnt, char **a)
{
    int i;
    int moreargs=1;

    for (i=1; i<cnt; i++)
    {
            if (moreargs && a[i][0] == '-')
            {
            char *p = &a[i][1];
            char c;
                if (*p == '-')
                {
                    moreargs = 0;
                    continue;
                }
                if (!strcmp(p,"long"))
                {
                    printf("Long argument\n");
                    i++;
                    continue;
                }

                while (c = *p)
                    switch (*p++)
                    {
                    case 'a':
                        printf("Option a\n");
                        break;

                    case 'b':
                        printf("Option b\n");
                        break;

                    case 'f':
                        if (!*p && i+1 < cnt)
                            printf("Value for f=%s\n", a[++i]);
                        else
                        {
                            printf("Value for f=%s\n", p);
                            while (*p)
                                p++;
                        }
                        break;

                        default:
                            printf("Bad switch %c\n",c);

            }
        }
    else
        process(a[i]);
    }
}



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Proper Case in a shell script

How do you force a string into proper case in a Unix shell script? (That is to say, capitalise the first letter and make the rest lower case). Bash4 has a special feature for doing it, but I’d avoid using it because, well, I want to be Unix/POSIX compatible.

It’s actually very easy once you’ve realised tr won’t do it all for you. The tr utility has no concept on where in the input stream it is, but combining tr with cut works a treat.

I came across this problem when I was writing a few lines to automatically create directory layouts for interpreted languages (in this case the Laminas framework). Languages of this type like capitalisation of class names, but other names have be lower case.

Before I get started, I note about expressing character ranges in tr. Unfortunately different systems have done it in different ways. The following examples assume BSD Unix (and POSIX). Unix System V required ranges to be in square brackets – e.g. A-Z becomes “[A-Z]”. And the quotes are absolutely necessary to stop the shell globing once you’ve introduced the square brackets!

Also, if you’re using a strange character set, consider using \[:lower:\] and \[:upper:\] instead of A-Z if your version of tr supports it (most do). It’s more compatible with foreign character sets although I’d argue it’s not so easy on the eye!

Anyway, these examples use A-Z to specify ASCII characters 0x41 to 0x5A – adjust to suit your tr if your Unix is really old.

To convert a string ($1) into lower case, use this:

lower=$(echo $1 | tr A-Z a-z)

To convert it into upper case, use the reverse:

upper=$(echo $1 | tr a-z A-Z)

To capitalise the first letter and force the rest to lower case, split using cut and force the first character to be upper and the rest lower:

proper=$(echo $1 | cut -c 1 | tr a-z A-Z)$(echo $1 | cut -c 2- | tr A-Z a-z)

A safer version would be:

proper=$(echo $1 | cut -c 1 | tr "[:lower:]" "[:upper:]")$(echo $1 | cut -c 2- | tr "[:upper:]" [":lower:"])

This is tested on FreeBSD in /bin/sh, but should work on all BSD and bash-based Linux systems using international character sets.

You could, if you wanted to, use sed to split up a multi-word string and change each word to proper case, but I’ll leave that as an exercise to the reader.

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Cookie problems

It’s been ten years since the original EU ePrivacy Directive (Regulation of the European Parliament and of the Council concerning the respect for private life and the protection of personal data in electronic communications and repealing Directive 2002/58/EC) came into effect in the UK. It’s implement as part of the equally wordy Privacy and Electronic Communications Regulations 2012 (PECR) One thing it did was require companies with websites to allow users to opt in to cookies. I’ve written about this before, but since then the amount of tracking cookies has become insane, and most people would choose to opt out of that much survailance.

The problem is that with so many cookies, some websites have effectively circumvent the law by making it impractical to opt out of all of them. At first glance they offer an easy way to turn everything off and “save settings”, but what’s not so clear is that they hide an individual option for every tracking cookie company they have a deal with. And there can be hundreds, each of which needs to be individually switched off using a mouse.

These extra cookies – and they’re the ones you don’t want – are usually hidden behind a “vendors” or “partners” tab. With the site shown below this was only found by scrolling all the way down and clicking on a link.

This kind of thing is not in the spirit of the act, and web sites that do this do not “care about your privacy” in any way, shape or form. And if you think these opt-in/out forms look similar, it’s because they are. Consent Management Platforms like Didomi, OneTrust and Quantcast are widely used to either set, or obfuscate what you’re agreeing to.

An update to the ePrivicy directive is now being talked about that says it must be “easy” to reject these tracking cookies, which isn’t the case now.

Meanwhile some governments are cracking down. In January, Google and Facebook both got slapped with huge fines in France from the
Commission Nationale de l’Informatique et des Libertés, which reckoned that because it took more clicks to reject cookies than accept them, Google and Facebook were not playing fair.

“Several clicks are required to refuse all cookies, against a single one to accept them. [The committee] considered that this process affects the freedom of consent: since, on the internet, the user expects to be able to quickly consult a website, the fact that they cannot refuse the cookies as easily as they can accept them influences their choice in favor of consent. This constitutes an infringement of Article 82 of the French Data Protection Act.”

I’m inclined to agree. And on top of a fines of €150 and €60 respectively, they’re being hit with €100K for each extra day they allow the situation to remain.

Unfortunately we’re not likely to see this in the UK. The EU can’t actually agree on the final form of the ePrivacy regulations. The UK, of course, is no longer in the EU and may be able to pass its own laws while the EU argues.

Last year the Information Commissioner’s office did start on this, taking proposals to the G7 in September 2021. Elizabeth Denham told the meeting that a popup with a button saying “I Agree” wasn’t exactly informed consent.

However, this year the government is going in the other direction. It’s just published plans to do away with the “cookie popup” and allow websites to slurp your data. Instead sites will be required to give users clear information on how to opt out. This doesn’t fill me with confidence.

Also in the proposals is to scrap the need for a Data Protection Impact Assessment (DPIA) before slurping data, replacing it with a “risk-based privacy management programme to mitigate the potential risk of protected characteristics not being identified”.

I don’t like the idea of any of this. However, there’s a better solution – simply use a web browser that rejects these tracking cookies in the first place.