Lxd: How to defragment ZFS ?

[vistoyn]

I try to use LXD on ZFS.

$ zpool list
NAME   SIZE  ALLOC   FREE  EXPANDSZ   FRAG    CAP  DEDUP  HEALTH  ALTROOT
lxd    408G   140G   268G         -    26%    34%  1.00x  ONLINE  -

$ zpool status
  pool: lxd
 state: ONLINE
  scan: none requested
config:

        NAME                    STATE     READ WRITE CKSUM
        lxd                     ONLINE       0     0     0
          /var/lib/lxd/zfs.img  ONLINE       0     0     0
          /data/index/file01    ONLINE       0     0     0
          /data/index/file02    ONLINE       0     0     0

errors: No known data errors

Size of file01, and file02 - 200G each file.

The fragmentation is 26%. How to defrag ZFS pool for LXD correctly?

[stgraber]

Not really a LXD question but a generic ZFS question.

I don't believe there is a way to do so. There is a plan that was mentioned a few times (block pointer rewrite), but fragmentation hasn't been enough of a problem for anyone using ZFS to cause someone to actually implement it. https://github.com/zfsonlinux/zfs/issues/3582

One thing to note is that the percentage you're seeing there is apparently free space fragmentation of your pool, not fragmentation of existing files in the pool.

[alek-p]

The way to defrag is to do a "zfs send" then "zfs recv" the pool but at 26% fragmentation, you probably don't have any performance impact due to fragmentation so why bother.