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Set up FreeBSD in two mirrored drives using UFS

I’ve written about the virtues of Geom Mirror (gmirror) in the past. Geom Mirror was probably the best way of implementing redundant storage between FreeBSD 5.3 (2004) until ZFS was introduced in FreeBSD 7.0 in 2008. Even then, ZFS is heavyweight and the Geom Mirror was tested and more practical for many years afterwards.

The Geom system also has a RAID3 driver. RAID3 is weird. It’s the one using a separate parity drive. It works, but it wasn’t popular. If you had a big FreeBSD system and wanted an array it was probably better to use an LSI host bus adapter and have that manage it with mptutil. But for small servers, especially remotely managed, Geom Mirror was the best. I’m still running it on a few twin-drive servers, and will probably continue for some time to come.

The original Unix File System (UFS2) actually has a couple of advantages over ZFS. Firstly it has much lower resource requirements. Secondly, and this is a big one, it has in-place updates. This is a big deal with random access files, such as databases or VM hard disks, as the Copy-on-Write system ZFS uses fragments the disk like crazy. To maintain performance on a massively fragmented file system, ZFS requires a huge amount of cache RAM.

What you need for random access read/write files are in-place updates. Database engines handle transaction groups themselves to ensure that the data structure’s integrity is maintained. ZFS does this at the file level instead of application level, which isn’t really good enough as the application knows what is and what isn’t required. There’s no harm in ZFS doing it too, but it’s a waste. And the file fragmentation is a high price to pay.

So, for database type applications, UFS2 still rules. There’s nothing wrong with having a hybrid system with both UFS and ZFS, even on the same disk. Just mount the UFS /var onto the ZFS tree.

But back to the twin drive system: The FreeBSD installed doesn’t have this as an option. So here’s a handy dandy script wot I rote to do it for you. Boot of a USB stick or whatever and run it.

Script to install FreeBSD on gmirror

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Use as much or as little as you like.

At the beginning of the script I define the two drives I will be using. Obviously change these! If the disks are not blank it might not work. The script tries to destroy the old partition data but you may need to do more if you have it set up with something unusual.

Be careful – it will delete everything on both drives without asking!

Read the comments in the script. I have set it up to use a 8g UFS partition, but if you leave out the “-s 8g” the final partition will use all the space, which is probably what you want. For debugging I kept it small.

I have put everything on a single UFS partition. If you want separate / /usr /var then you need to modify it to what you need and create a mirror for each (and run newfs for each). The only think is that I’ve created a swap file on each drive that is NOT mirrored and configured it to use both.

I have not set up everything on the new system, but it will boot and you can configure other stuff as you need by hand. I like to connect to the network and have an admin user so I can work on a remote terminal straight away, so I have created an “admin” user with password “password” and enabled the ssh daemon. As you probably know, FreeBSD names its Ethernet adapters by manufacturer and you don’t know what you’ll have so I just have it try DHCP on every possible interface. Edit the rc.conf file how you need it once it’s running.

If base.txz and kernel.txz are in the current directory, fine. The script tries to download them at present.

And finally, I call my mirrors m0, m1, m2 and so on. Some people like to use gm0. It really doesn’t matter what you call them. 

#!/bin/sh
# Install FreeBSD on two new disks set up a a gmirror
# FJL 2025
# Edit stuff in here as needed. At present it downloads
# FreeBSD 14.2-RELEASE and assumes the disks
# in use are ada0 and ada1

# Fetch the OS files if needed (and as appropriate)
fetch https://download.freebsd.org/ftp/releases/amd64/14.2-RELEASE/kernel.txz
fetch https://download.freebsd.org/ftp/releases/amd64/14.2-RELEASE/base.txz

# Disks to use for a mirror. All will be destroyed! Edit these. The -xxxx 
# is there to save you if you don't
D0=/dev/da1-xxxxx 
D1=/dev/da2-xxxxx 

# User name and password to set up initial user.
ADMIN=admin 
ADMINPASS=password 

# Make sure the geom mirror module is loaded.
kldload geom_mirror 

# Set up the first drive
 echo Clearing $D0 
 gpart destroy -F $D0 
 dd if=/dev/zero of=$D0 bs=1m count=10 

# Then create p1 (boot), p2 (swap) and p3 (ufs)
# Note the size of the UFS partition is set to 8g. If you delete
# the -s 8g it will use the rest of the disk by default. For testing
# it's better to have something small so newfs finishes quick.

echo Creating gtp partition on $D0 
gpart create -s gpt $D0 
gpart add -t freebsd-boot -s 512K $D0 
gpart add -t freebsd-swap -s 4g $D0 
gpart add -t freebsd-ufs -s 8g $D0 

echo Installing boot code on $D0 
# -b installs protective MBR, -i the Bootloader.
# Assumes partition 1 is freebsd-boot created above.
gpart bootcode -b /boot/pmbr -p /boot/gptboot -i 1 $D0 

# Set up second drive
echo Clearing $D1 
gpart destroy -F $D1 
dd if=/dev/zero of=$D1 bs=1m count=10 

# Copy partition data to second drive and put on boot code
gpart backup $D0 | gpart restore $D1 
gpart bootcode -b /boot/pmbr -p /boot/gptboot -i 1 $D1 

# Mirror partition 3 on both drives
gmirror label -v m0 ${D0}p3 ${D1}p3 

echo Creating file system 
newfs -U /dev/mirror/m0 
mkdir -p /mnt/freebsdsys 
mount  /dev/mirror/m0 /mnt/freebsdsys 

echo Decompressing Kernel 
tar -x -C /mnt/freebsdsys -f kernel.txz 
echo Decompressing Base system 
tar -x -C /mnt/freebsdsys -f base.txz 

# Tell the loader where to mount the root system from
echo 'geom_mirror_load="YES"' > /mnt/freebsdsys/boot/loader.conf 
echo 'vfs.root.mountfrom="ufs:/dev/mirror/m0"' \
       >> /mnt/freebsdsys/boot/loader.conf 

# Set up fstab so it all mounts.
echo $D0'p2 none swap sw 0 0' > /mnt/freebsdsys/etc/fstab 
echo $D1'p2 none swap sw 0 0' >> /mnt/freebsdsys/etc/fstab 
echo '/dev/mirror/m0 / ufs rw 1 1' >> /mnt/freebsdsys/etc/fstab 

# Enable sshd and make ethernet interfaces DHCP configure
echo 'sshd_enable="YES"' >/mnt/freebsdsys/etc/rc.conf 
for int in em0 igb0 re0 bge0 alc0 fxp0 xl0 ue0 igb0 xcgbe0 bnxt0 mlx0 
do 
    echo 'ifconfig_'$int'="DHCP"' >>/mnt/freebsdsys/etc/rc.conf 
done 

# Create initial user suitable for ssh login
pw -R /mnt/freebsdsys useradd $ADMIN -G wheel -m 
echo "$ADMINPASS" | pw -R /mnt/freebsdsys usermod -n $ADMIN -h 0 
echo "$ADMINPASS" | openssl passwd -6 -stdin | pw -R /mnt/freebsdsys usermod -n $ADMIN -H 0

# Tidy up
umount /mnt/freebsdsys 
echo Done. Remove USB stick or whatever and reboot.

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ZFS is not always the answer. Bring back gmirror!





The ZFS bandwaggon has momentum, but ZFS isn’t for everyone. UFS2 has a number of killer advantages in some applications.

ZFS is great if you want to store a very large number of normal files safely. It’s copy-on-write (COW) is a major advantage for backup, archiving and general data safety, and datasets allow you to fine-tune almost any way you can think of. However, in a few circumstances, UFS2 is better. In particular, large random-access files do badly with COW.

Unlike traditional systems, a block in a file isn’t overwritten in place, it always ends up at a different location. If a file started off contiguous it’ll pretty soon be fragmented to hell and performance will go off a cliff. Obvious victims will be databases and VM hard disk images. You can tune for these, but to get acceptable performance you need to throw money and resources to bring ZFS up to the same level. Basically you need huge RAM caches, possibly an SLOG, and never let your pool get more than 50% full. If you’re unlucky enough to end up at 80% full ZFS turns off speed optimisations to devote more RAM to caching as things are going to get very bad fragmentation-wise.

If these costs are a problem, stuck with UFS. And for redundancy, there is still good old GEOM Mirror (gmirror). Unfortunately the documentation of this now-poor relation has lagged a bit, and what once worked as standard, doesn’t. So here are some tips.

The most common use of gmirror (with me anyway) is a twin-drive host. Basically I don’t want things to fail when a hard disk dies, so I add a second redundant drive. Such hosts (often 1U servers) don’t have space for more than two drives anyway – and it pays to keep things simple.

Setting up a gmirror is really simple. You create one using the “gmirror label” command. There is no “gmirror create” command; it really is called “label”, and it writes the necessary metadata label so that mirror will recognise it (“gmirror destroy” is present and does exactly what you might expect).

So something like:

gmirror label gm0 ada1 ada2

will create a device called /dev/mirror/gm0 and it’ll contain ada1’s contents mirrored on to ada2 (once it’s copied it all in the background). Just use /dev/mirror/gm0 as any other GEOM (i.e. disk). Instead of calling it gm0 I could have called it gm1, system, data, flubnutz or anything else that made sense, but gm0 is a handy reminder that it’s the first geom mirror on the system and it’s shorter to type.

The eagle eyed might have noticed I used ada1 and ada2 above. You’ve booted off ada0, right? So what happens if you try mirroring yourself with “gmirror label gm0 ada0 ada1“? Well this used to work, but in my experience it doesn’t any more. And on a twin-drive system, this is exactly what you want to do. But it is still possible, read on…

How to set up a twin-drive host booting from a geom mirror

First off, before you do anything (even installing FreeBSD) you need to set up your disks. Since the IBM XT, hard disks have been partitioned using an MBR (Master Boot Record) at the start. This is really old, naff, clunky and Microsoft. Those in the know have been using the far superior GPT system for ages, and it’s pretty cross-platform now. However, it doesn’t play nice with gmirror, so we’re going to use MBR instead. Trust me on this.

For the curious, know that GPT keeps a copy of the partition table at the beginning and end of the disk, but MBR only has one, stored at the front. gmirror keeps its metadata at the end of the disk, well away from the MBR but unfortunately in exactly the same spot as the spare GPT. You can hack the gmirror code so it doesn’t do this, or frig around with mirroring geoms rather than whole disks and somehow get it to boot, but my advice is to stick to MBR partitioning or BSDlabels, which is an extension. There’s not a lot of point in ever mounting your BSD boot drive on a non-BSD system, so you’re not losing much whatever you choose.

Speaking of metadata, both GPT and gmirror can get confused if they find any old tables or labels on a “new” disk. GPT will find old backup partition tables and try to restore them for you, and gmirror will recognise old drives as containing precious data and dig its heels in when you try to overwrite it. Both gpart and gmirror have commands to erase their metadata, but I prefer to use dd to overwrite the whole disk with zeros anyway before re-use. This checks that the disk is actually good, which is nice to know up-front. You could just erase the start and end if you were in a hurry and wanted to calculate the offsets.

The next thing you’ll need to do is load the geom_mirror kernel module. Either recompile the kernel with it added, or if this fills you with horror,  just add ‘load_geom_mirror=”yes”‘ to /boot/loader.conf. This does bring it in early enough in the process to let you boot from it. The loader will boot from one drive or the other and then switch to mirror mode when it’s done.

So, at this point, you’ve set up FreeBSD as you like on one drive (ada0), selecting BSDlabels or MBR as the partition method and UFS as the file system. You’ve set it to load the geom_mirror module in loader.conf.  You’re now looking at a root prompt on the console, and I’m assuming your drives are ada0 and ada1, and you want to call your mirror gm0.

Try this:

gmirror label gm0 ada0

Did it work? Well it used to once, but now you’ll probably get an error message saying it could not write metadata to ada0. If (when) this happens I know of one answer, which I found after trying everything else. Don’t be tempted to try everything else yourself (such as seeing if it works with ada1). Anything you do will either fail if you’re lucky, or make things worse. So just reboot, and select single-user mode from the loader menu.

Once you’re at the prompt, type the command again, and this time it should say that gm0 is created. My advice is to now reboot rather than getting clever.

When you do reboot it will fail to mount the root partition and stop, asking for help to find it. Don’t panic. We know where it’s gone. Mount it with “ufs:/dev/mirror/gm0s1a” or whatever slice you had it on if you’ve tried to be clever. Forgot to make a note? Don’t worry, somewhere on the boot long visible on the screen it actually tell you the name of the partition it couldn’t find.

After this you should be “in”. And to avoid this inconvenience next time you boot you’ll need to tweak /etc/fstab using an editor of your choice, although real computer nerds only use vi. What you need to do is replace all references to the actual drive with the gm0 version. Therefore /dev/ada0s1a should be edited to read /dev/mirror/gm0s1a. On a current default install, which no longer partitions the drive, this will only apply the root mount point and the swap file.

Save this, reboot (to test) and you should be looking good. Now all that remains is to add the second drive (ada1 in the example) with the line:

gmirror insert gm0 ada1

You can see the effect by running:

gmirror status

Unless your drive is very small, gm0 will be DEGRADED and it will say something about being rebuilt. The precise wording has changed over time. Rebuilding takes hours, not seconds so leave it. Did I mention it’s a good idea to do this when the system isn’t busy?