Transparent compression filesystem in conjunction with ext4

I use ZFS on Linux as a volume manager and a means to provide additional protections and functionality to traditional filesystems. This includes bringing block-level snapshots, replication, deduplication, compression and advanced caching to the XFS or ext4 filesystems.

See: https://pthree.org/2012/12/21/zfs-administration-part-xiv-zvols/ for another explanation.

In my most common use case, I leverage the ZFS zvol feature to create a sparse volume on an existing zpool. That zvol's properties can be set just like a normal ZFS filesystem's. At this juncture, you can set properties like compression type, volume size, caching method, etc.

Creating this zvol presents a block device to Linux that can be formatted with the filesystem of your choice. Use fdisk or parted to create your partition and mkfs the finished volume.

Mount this and you essentially have a filesystem backed by a zvol and with all of its properties.


Here's my workflow...

Create a zpool comprised of four disks:
You'll want the ashift=12 directive for the type of disks you're using. The zpool name is "vol0" in this case.

zpool create -o ashift=12 -f vol0 mirror scsi-AccOW140403AS1322043 scsi-AccOW140403AS1322042 mirror scsi-AccOW140403AS1322013 scsi-AccOW140403AS1322044

Set initial zpool settings:
I set autoexpand=on at the zpool level in case I ever replace the disks with larger drives or expand the pool in a ZFS mirrors setup. I typically don't use ZFS raidz1/2/3 because of poor performance and the inability to expand the zpool.

zpool set autoexpand=on vol0

Set initial zfs filesystem properties:
Please use the lz4 compression algorithm for new ZFS installations. It's okay to leave it on all the time.

zfs set compression=lz4 vol0
zfs set atime=off vol0

Create ZFS zvol:
For ZFS on Linux, it's very important that you use a large block size. -o volblocksize=128k is absolutely essential here. The -s option creates a sparse zvol and doesn't consume pool space until it's needed. You can overcommit here, if you know your data well. In this case, I have about 444GB of usable disk space in the pool, but I'm presenting an 800GB volume to XFS.

zfs create -o volblocksize=128K -s -V 800G vol0/pprovol

Partition zvol device:
(should be /dev/zd0 for the first zvol; /dev/zd16, /dev/zd32, etc. for subsequent zvols)

fdisk /dev/zd0 # (create new aligned partition with the "c" and "u" parameters)

Create and mount the filesystem:
mkfs.xfs or ext4 on the newly created partition, /dev/zd0p1.

mkfs.xfs -f -l size=256m,version=2 -s size=4096 /dev/zd0p1

Grab the UUID with blkid and modify /etc/fstab.

UUID=455cae52-89e0-4fb3-a896-8f597a1ea402 /ppro       xfs     noatime,logbufs=8,logbsize=256k 1 2

Mount the new filesystem.

mount /ppro/

Results...

[root@Testa ~]# df -h
Filesystem      Size  Used Avail Use% Mounted on
/dev/sde2        20G  8.9G  9.9G  48% /
tmpfs            32G     0   32G   0% /dev/shm
/dev/sde1       485M   63M  397M  14% /boot
/dev/sde7       2.0G   68M  1.9G   4% /tmp
/dev/sde3        12G  2.6G  8.7G  24% /usr
/dev/sde6       6.0G  907M  4.8G  16% /var
/dev/zd0p1      800G  398G  403G  50% /ppro  <-- Compressed ZFS-backed XFS filesystem.
vol0            110G  256K  110G   1% /vol0

ZFS filesystem listing.

[root@Testa ~]# zfs list
NAME           USED  AVAIL  REFER  MOUNTPOINT
vol0           328G   109G   272K  /vol0
vol0/pprovol   326G   109G   186G  -   <-- The actual zvol providing the backing for XFS.
vol1           183G   817G   136K  /vol1
vol1/images    183G   817G   183G  /images

ZFS zpool list.

[root@Testa ~]# zpool list -v
NAME   SIZE  ALLOC   FREE    CAP  DEDUP  HEALTH  ALTROOT
vol0   444G   328G   116G    73%  1.00x  ONLINE  -
  mirror   222G   164G  58.1G         -
    scsi-AccOW140403AS1322043      -      -      -         -
    scsi-AccOW140403AS1322042      -      -      -         -
  mirror   222G   164G  58.1G         -
    scsi-AccOW140403AS1322013      -      -      -         -
    scsi-AccOW140403AS1322044      -      -      -         -

ZFS zvol properties (take note of referenced, compressratio and volsize).

[root@Testa ~]# zfs get all vol0/pprovol
NAME          PROPERTY               VALUE                  SOURCE
vol0/pprovol  type                   volume                 -
vol0/pprovol  creation               Sun May 11 15:27 2014  -
vol0/pprovol  used                   326G                   -
vol0/pprovol  available              109G                   -
vol0/pprovol  referenced             186G                   -
vol0/pprovol  compressratio          2.99x                  -
vol0/pprovol  reservation            none                   default
vol0/pprovol  volsize                800G                   local
vol0/pprovol  volblocksize           128K                   -
vol0/pprovol  checksum               on                     default
vol0/pprovol  compression            lz4                    inherited from vol0
vol0/pprovol  readonly               off                    default
vol0/pprovol  copies                 1                      default
vol0/pprovol  refreservation         none                   default
vol0/pprovol  primarycache           all                    default
vol0/pprovol  secondarycache         all                    default
vol0/pprovol  usedbysnapshots        140G                   -
vol0/pprovol  usedbydataset          186G                   -
vol0/pprovol  usedbychildren         0                      -
vol0/pprovol  usedbyrefreservation   0                      -
vol0/pprovol  logbias                latency                default
vol0/pprovol  dedup                  off                    default
vol0/pprovol  mlslabel               none                   default
vol0/pprovol  sync                   standard               default
vol0/pprovol  refcompressratio       3.32x                  -
vol0/pprovol  written                210M                   -
vol0/pprovol  snapdev                hidden                 default

You also need to enable discard on the ext4 filesystem. Without discard, zfs does not reclaim the space when files are removed. This can end up leading to large space discrepancies between what the ext4 filesystem reports and the zfs volume reports.