Alexander Leidinger

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Another root-on-zfs HOWTO (opti­mized for 4k-sector drives)

After 9 years with my cur­rent home-server (one jail for each ser­vice like MySQL, Squid, IMAP, Web­mail, …) I decided that it is time to get some­thing more recent (spe­cially as I want to install some more jails but can not add more mem­ory to this i386 system).

With my old sys­tem I had an UFS2-root on a 3-way-gmirror, swap on a 2-way-gmirror and my data in a 3-partition raidz (all in dif­fer­ent slices of the same 3 hard­disks, the 3rd slice which would cor­re­spond to the swap was used as a crash­dump area).

For the new sys­tem I wanted to go all-ZFS, but I like to have my boot area sep­a­rated from my data area (two pools instead of one big pool). As the machine has 12 GB RAM I also do not con­fig­ure swap areas (at least by default, if I really need some swap I can add some later, see below). The sys­tem has five 1 TB hard­disks and a 60 GB SSD. The hard­disks do not have 4k-sectors, but I expect that there will be more and more 4k-sector dri­ves in the future. As I pre­fer to plan ahead I installed the ZFS pools in a way that they are “4k-ready”. For those which have 4k-sector dri­ves which do not tell the truth but announce they have 512 byte sec­tors (I will call them pseudo-4k-sector dri­ves here) I include a descrip­tion how to prop­erly align the (GPT-)partitions.

A major require­ment to boot 4k-sector-size ZFS pools is ZFS v28 (to be cor­rect here, just the boot-code needs to sup­port this, so if you take the pmbr and gptzfs­boot from a ZFS v28 sys­tem, this should work… but I have not tested this). As I am run­ning 9-current, this is not an issue for me.

A quick descrip­tion of the task is to align the partition/slices prop­erly for pseudo-4k-sector dri­ves, and then use gnop tem­po­rary dur­ing pool cre­ation time to have ZFS use 4k-sectors dur­ing the life­time of the pool. The long descrip­tion follows.

The lay­out of the drives

The five equal dri­ves are par­ti­tioned with a GUID par­ti­tion table (GPT). Each drive is divided into three par­ti­tions, one for the boot code, one for the root pool, and one for the data pool. The root pool is a 3-way mir­ror and the data pool is a raidz2 pool over all 5 disks. The remain­ing space on the two hard­disks which do not take part in the mir­ror­ing of the root pool get swap par­ti­tions of the same size as the root par­ti­tions. One of them is used as a dumpde­vice (this is –cur­rent, after all), and the other one stays unused as a cold-standby. The 60 GB SSD will be used as a ZFS cache device, but as of this writ­ing I have not decided yet if I will use it for both pools or only for the data pool.

Cal­cu­lat­ing the offsets

The first sec­tor after the GPT (cre­ated with stan­dard set­tings) which can be used as the first sec­tor for a par­ti­tion is sec­tor 34 on a 512 bytes-per-sector drive. On a pseudo-4k-sector drive this would be some­where in the sec­tor 4 of a real 4k-sector, so this is not a good start­ing point. The next 4k-aligned sec­tor on a pseudo-4k-sector drive is sec­tor 40 (sec­tor 5 on a real 4k-sector drive).

The first par­ti­tion is the par­ti­tion for the FreeBSD boot code. It needs to have enough space for gptzfs­boot. Only allo­cat­ing the space needed for gptzfs­boot looks a lit­tle bit dan­ger­ous regard­ing future updates, so my hard­disks are con­fig­ured to allo­cate half a megabyte for it. Addi­tion­ally I leave some unused sec­tors as a safety mar­gin after this first partition.

The sec­ond par­ti­tion is the root pool (respec­tively the swap par­ti­tions). I let it start at sec­tor 2048, which would be sec­tor 256 on a real 4k-sector drive (if you do not want to waste less than half a megabyte just cal­cu­late a lower start sec­tor which is divis­i­ble by 8 (-> start % 8 = 0)). It is a 4 GB par­ti­tion, this is enough for the basesys­tem with some debug ker­nels. Every­thing else (/usr/{src,ports,obj,local}) will be in the data partition.

The last par­ti­tion is directly after the sec­ond and uses the rest of the hard­disk rounded down to a full GB (if the disk needs to be replaced with a sim­i­lar sized disk there is some safety mar­gin left, as the num­ber of sec­tors in hard­disks fluc­tu­ates a lit­tle bit even in the same mod­els from the same man­u­fac­tur­ing charge). For my hard­disks this means a lit­tle bit more than half a giga­byte of wasted stor­age space.

The com­mands to par­ti­tion the disks

In the fol­low­ing I use ada0 as the device of the disk, but it also works with daX or adX or sim­i­lar. I installed one disk from an exist­ing 9-current sys­tem instead of using some kind of instal­la­tion media (beware, the pool is linked to the sys­tem which cre­ates it, I booted a life-USB image to import it on the new sys­tem and copied the zpool.cache to /boot/zfs/ after import­ing on the new system).

Cre­ate the GPT:

gpart create -s gpt ada0

Cre­ate the boot partition:

gpart add -b 40 -s 1024 -t freebsd-boot ada0

Cre­ate the root/swap par­ti­tions and name them with a GPT label:

gpart add -b 2048 -s 4G -t freebsd-zfs -l rpool0 ada0

or for the swap

gpart add -b 2048 -s 4G -t freebsd-swap -l swap0 ada0

Cre­ate the data par­ti­tion and name them with a GPT label:

gpart add -s 927G -t freebsd-zfs -l data0 ada0

Install the boot code in par­ti­tion 1:

gpart bootcode -b /boot/pmbr -p /boot/gptzfsboot -i 1 ada0

The result looks like this:

# gpart show ada0
=>        34  1953525101  ada0  GPT  (931G)
          34           6        - free -  (3.0k)
          40        1024     1  freebsd-boot  (512k)
        1064         984        - free -  (492k)
        2048     8388608     2  freebsd-zfs  (4.0G)
     8390656  1944059904     3  freebsd-zfs  (927G)
  1952450560     1074575        - free -  (524M)

Cre­ate the pools with 4k-ready inter­nal structures

Cre­at­ing a ZFS pool on one of the ZFS par­ti­tions with­out prepa­ra­tion will not cre­ate a 4k-ready pool on a pseudo-4k-drive. I used gnop (the set­tings do not sur­vive a reboot) to make the par­ti­tion tem­po­rary a 4k-sector par­ti­tion (only the com­mand for the root pool is shown, for the data par­ti­tion gnop has to be used in the same way):

gnop create -S 4096 ada0p2
zpool create -O utf8only=on -o failmode=panic rpool ada0p2.nop
zpool export rpool
gnop destroy ada0p2.nop
zpool import rpool

After the pool is cre­ated, it will keep the 4k-sectors set­ting, even when accessed with­out gnop. You can ignore the options I used to cre­ate the pool, they are just my pref­er­ences (and the utf8only set­ting can only be done at pool cre­ation time). If you pre­pare this on a sys­tem which already has a zpool on its own, you can maybe spec­ify “-o cachefile=/boot/zfs/zpool2.cache” and copy it to the new pool as zpool.cache to make it bootable with­out the need of a life-image for the new sys­tem (I did not test this).

Ver­i­fy­ing if a pool is pseudo-4k-ready

To ver­ify that the pool will use 4k-sectors, you can have a look at the ashift val­ues of the pool (the ashift is per vdev, so if you e.g. con­cat­te­nate sev­eral mir­rors, the ashift needs to be ver­i­fied for each mir­ror, and if you con­cat­te­nate just a bunch of disks, the ashift needs to be ver­i­fied for all disks). It needs to be 12. To get the ashift value you can use zdb:

zdb rpool | grep ashift

Set­ting up the root pool

One of the ben­e­fits of root-on-zfs is that I can have mul­ti­ple FreeBSD boot envi­ron­ments (BE). This means that I not only can have sev­eral dif­fer­ent ker­nels, but also sev­eral dif­fer­ent user­land ver­sions. To han­dle them com­fort­ably, I use man­ageBE from Philipp Wuen­sche. This requires a spe­cific setup of the root pool:

zfs create rpool/ROOT
zfs create rpool/ROOT/r220832M
zpool set bootfs=rpool/ROOT/r220832M rpool
zfs set freebsd:boot-environment=1 rpool/ROOT/r220832M   # manageBE setting

The r220832M is my ini­tial BE. I use the SVN revi­sion of the source tree which was used dur­ing install of this BE as the name of the BE here. You also need to add the fol­low­ing line to /boot/loader.conf:


As I want to have a shared /var and /tmp for all my BEs, I cre­ate them separately:

zfs create -o exec=off -o setuid=off -o mountpoint=/rpool/ROOT/r220832M/var rpool/var
zfs create -o setuid=off -o mountpoint=/rpool/ROOT/r220832M/tmp rpool/tmp

As I did this on the old sys­tem, I did not set the mount­points to /var and /tmp, but this has to be done later.

Now the user­land can be installed (e.g. buildworld/installworld/buildkernel/buildkernel/mergemaster with DESTDIR=/rpool/ROOT/r220832M/, do not for­get to put a good master.passwd/passwd/group in the root pool).

When the root pool is ready make sure an empty /etc/fstab is inside, and con­fig­ure the root as fol­lows (only show­ing what is nec­es­sary for root-on-zfs):



At this point of the setup I unmounted all zfs on rpool, set the mount­point of rpool/var to /var and of rpool/tmp to /tmp, exported the pool and installed the hard­disk in the new sys­tem. After boot­ing a life-USB-image, import­ing the pool, putting the result­ing zpool.cache into the pool (rpool/ROOT/r220832M/boot/zfs/), I rebooted into the rpool and attached the other hard­disks to the pool (“zpool attach rpool ada0p2 ada1p2”, “zpool attach rpool ada0p2 ada2p2”):

After updat­ing to a more recent ver­sion of 9-current, the BE looks like this now:

# ./bin/manageBE list
Poolname: rpool
BE                Active Active Mountpoint           Space
Name              Now    Reboot -                    Used
----              ------ ------ ----------           -----
r221295M          yes    yes    /                    2.66G
cannot open '-': dataset does not exist
r221295M@r221295M no     no     -
r220832M          no     no     /rpool/ROOT/r220832M  561M

Used by BE snapshots: 561M

The lit­tle bug above (the error mes­sage which is prob­a­bly caused by the snap­shot which shows up here prob­a­bly because I use listsnapshots=on) is already reported to the author of manageBE.


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