ZFS
Jurgen Weber
ZFS is an exciting new file system developed by Sun and recently ported to FreeBSD. Many people are excited by the possibilities of ZFS (including us) as it promises to simplify a great many things. It offers:
• increased reliability through checksums, multiple copies of data and self-healing RAID
• elimination of that dreaded “oh, if only I’d made the /var partition larger” feeling. Partitions can now be resized at any time and in fact can each be allocated up to the full size of the storage media.
• built-in compression and encryption
• built-in NFS file sharing
• clean, easy to use toolset for creating storage pools, volumes and much more
• snapshots and rollbacks for backups
Read more about some of the powerful commands here
Installing ZFS on FreeBSD
Since it is early days for ZFS on BSD, the installer doesn’t yet support ZFS natively. So there are a few tricks to getting it up and running. Also, you cannot boot directly from a ZFS partition since adding that functionality to the boot loader in FreeBSD is a huge undertaking.
Step One: installing FreeBSD
• Boot up on the FreeBSD Current 7.0 CD
• Choose Country and Locale
• Go Custom install
• Partition fdisk with Auto option (one slice, whole disk)
• Disklabel with options
• A: 1024Mb UFS2 /
• B: swap
D: rest of disk
To create D you’ll need to enter any mount point you want and then use the M option to clear it. This ensures that it will not mount or be created as a file system.
• Distribution choose Minimal install
• Media Select CD/DVD
System will install to the small 1024MB UFS root partition you created.
Step Two: creating the ZFS pool
• Once installation is complete, remove CD and reboot into our new FreeBSD system.
• Boot into FreeBSD partition and select “4” for single user mode.
• Hit ENTER to accept /bin/sh shell.
# mount -w /
Now create a Disk pool using the D label we prepared during the install. In this example we have a SATA disk at ad4.
# zpool create tank /dev/ad4s1d
Firstly stop ZFS from creating default mountpoints for shares
# zfs set mountpoint=none tank
Create some extra/common mountpoints
# zfs create tank/root
# zfs create tank/usr
# zfs create tank/var
# zfs create tank/tmp
Now set the mountpoints of the shares
# zfs set mountpoint=/tank tank/root
# zfs set mountpoint=/tank/usr tank/usr
# zfs set mountpoint=/tank/var tank/var
# zfs set mountpoint=/tank/tmp tank/tmp
Have a look to what we have done
#df -h
#zfs list
Beautiful isn’t it? :)
Edit /etc/rc.conf and enable ZFS
# echo 'zfs_enable="YES"' >> /etc/rc.conf
Now copy the UFS bootable slice to the ZFS mountpoint. This gives us a workable FreeBSD installation under ZFS.
# find -x / | cpio -pmd /tank
(you can ignore any errors here if you get them)
Step Three: solving the ZFS boot problem
The problem which remains is that FreeBSD will not be able to boot directly into ZFS since the bootloader doesn’t know anything about ZFS. So a little trick is that we put the kernel onto /boot which lives on the UFS partition. This gets the system running to the point where ZFS can be mounted and the rest of the boot proceeds.
Remove the /tank/boot just copied over from the UFS system:
# rm -rf /tank/boot
Now make the directory in which our UFS partition will be mounted. This will be useful later on when we want to update the contents of that slice from a running system. We also need to make sure that when the ZFS is booting up it can see the UFS bootdir.
# mkdir /tank/bootdir
# cd /tank
# ln -s bootdir/boot boot
Now we tell the loader on the UFS slice to load and boot from the contents of the ZFS volume:
# echo 'zfs_load="YES"' >> /boot/loader.conf
# echo 'vfs.root.mountfrom="zfs:tank/root"' >> /boot/loader.conf
Edit /tank/etc/fstab so our UFS slice is mounted in the right location for when ZFS boots.
/dev/ad4s1a /bootdir ufs rw 1 1
When we reboot in a minute, we want the ZFS tank to mount in /var /usr /tmp and/ and not within the /tank location it is now. So set the true mountpoints:
# zfs set mountpoint=/tmp tank/tmp
# zfs set mountpoint=/usr tank/usr
# zfs set mountpoint=/var tank/var
Set root mount point to ‘legacy’ so ZFS won’t try to mount it automatically. It should already have been mounted by the loader:
# cd /
# zfs set mountpoint=legacy tank/root
All Done!
Reboot and login as root. df -h and zfs list and you will see everything.
# df -h
Filesystem Size Used Avail Capacity Mounted on
tank/root 282G 454M 281G 0% /
devfs 1.0K 1.0K 0B 100% /dev
/dev/ad4s1a 496M 317M 139M 69% /bootdir
tank/tmp 281G 1.1M 281G 0% /tmp
tank/usr 287G 5.7G 281G 2% /usr
tank/var 281G 76M 281G 0% /var
# zfs list
NAME USED AVAIL REFER MOUNTPOINT
tank 6.27G 281G 454M none
tank/root 16.2M 51.3G 16.2M legacy
tank/tmp 1.10M 281G 1.10M /tmp
tank/usr 5.75G 281G 5.75G /usr
tank/var 75.5M 281G 75.5M /var
All is done. :) Basic install completed. At this point you might want to install a more complete version of FreeBSD, install the ports system or anything else you normally do.
Creating a Mirror
That was pretty exciting.
Now let’s say that we have some important data on that drive and we are pretty keen not to lose it. So, let’s add another drive as a mirror under ZFS. In some ways this is even superior to a hardware RAID setup since ZFS is able to monitor the checksums on the disk and automatically detect which of the two drives has corrupted a block of data, transparently using the other and repairing if necessary.
• Install the HDD and boot up on the system.
• Login as root
• Run sysinstall
• Enter Custom Install
• Partition fdisk with Auto option (one slice, who disk) and press ‘w’ to write changes
• Label with options
• A: 1024Mb / (you must press M and remove the mount point as you have done previously)
• B: swap
D: rest of disk empty (for zfs later)
and once again, press ‘w’ to save changes and state ‘YES’ to the question it asks you about applying now. You’ll need to make this identical to the labels on the first drive.
• Exit sysinstall.
Check out what we have:
#zpool status
Add the 2nd drive to our mirror (our second drive comes up as ad6)
# zpool attach tank ad4s1d ad6s1d
# zpool status
and it will now show the two HDD’s in a mirror
pool: tank
state: ONLINE
scrub: none requested
config:
NAME STATE READ WRITE CKSUM
tank ONLINE 0 0 0
mirror ONLINE 0 0 0
ad4s1d ONLINE 0 0 0
ad6s1d ONLINE 0 0 0
errors: No known data errors
Now we have a mirror, but if we lose the first drive we will need the contents of the UFS slice copied to the second drive in order to reboot.
# newfs /dev/ad6s1a
# mkdir /mnt/bootdir
# mount /dev/ad6s1a /mnt/bootdir
# find -x /bootdir | cpio -pmd /mnt
This gives us another ready to use system on the 2nd HDD in case we have a failure on the 1st HDD with all the necessary zfs tools to move forward.
We also need to stop fsck occurring during the boot process for the bootdir so if the first HDD fails (ad4) the system will continue to boot without issue. This means the bootup completes as follows: The first HDD (AD4) is not present in the system, the BIOS will hand over to the MBR on the 2nd HDD (AD6) this will load all the zfs modules and the system will complete its bootup as normal. For this to always function you NEED to make sure the /bootdir/zfs/zpool.cache file is the same on both HDDs.
Edit /etc/fstab so our UFS slice is mounted in the right location for when ZFS boots.
/dev/ad4s1a /bootdir ufs rw 1 0
/dev/ad6s1a /mnt/bootdir ufs rw 1 0
The /bootdir is only needed for when you make changes to your zfs pools, etc
/boot/Loader.conf tweaks
As you know, ZFS within Freebsd is and Experimental system and there are still some bugs to iron out. In particular, it is easy for ZFS to run out of memory and crash the kernel. It helps to place the following in /boot/loader.conf (and if you completed the above make sure you change /mnt/bootdir/boot/loader.conf as well).
vm.kmem_size_max=“512M”
vm.kmem_size=“512M”
vfs.zfs.zil_disable=1
The amount for the kmem size is up to you but if you make it too large (something around 1500M) you will kernel panic on boot up. Tuning memory usage for ZFS appears to be an art rather than a science right now, but some useful tips exist.
Disaster recovery
(Maybe necessary if you have just completed the before mentioned tweaks and rebooted)
With the way we have setup our system if the ZFS fails to boot or function for whatever reason we can still boot off the original minimal install at any time. To do this do the following:
• Boot up
• Choose #6 (Escape to Loader Prompt) at the boot menu.
• Suppress the mounting of the ZFS volumes like this and disable the zfs module:
• # unset vfs.root.mountfrom
• # disable-module zfs
• Suppress of kmem sizes
• # unset vm.kmem_size
• # unset vm.kmem_size_max
• Single user mode and boot
• # set boot_single
• # boot
• Login
and edit the /boot/loader.conf as necessary
# df -h
# zpool list
You will notice that it has not picked up the ZFS drives or the pool. Never fear we can import it!
#zpool import -d /boot/zfs
This loads the zpool data so the libraries can find the information it needs, now we need to import our pool using an alternate root.
#zpool import -f -R /tank tank
And check out that it has worked.
# df -h
# zpool list
Useful Commands
Create a mirror
#zpool create -m /usr/local/www/ mirror
Show available pool/resources
#zfs list
Show Status of Disk/pool
#zpool status
Changing Mount points for created resources
#zfs set mountpoint=/
Data Limiting
# zfs set quota=10g testpool/testfs/dir1
(logically limits space)
# zfs set reservation=20g testpool/testfs/dir2
(logically preallocates space)
Unloading a kernel and choosing another one (in the boot loader mode)
# unload kernel
# boot /boot/kernel.old/kernel
Trouble shooting
To see ZFS debug messages on the console you can add vfs.zfs.debug=1 in the /boot/loader.conf
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