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Asked: 2016-08-24 01:14:24 +0800 CST

Why is `zfs list -t snapshot` orders of magnitude slower than `ls .zfs/snapshot`?

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With all ZFS-on-Linux versions I've ever tried, using zfs list to list all snapshots of a filesystem or volum (zfs list -r -t snapshot -H -o name pool/filesystem) always takes many orders of magnitude more time to run than ls .zfs/snapshot, which is immediate:

$ time ls -1 /srv/vz/subvol-300-disk-1/.zfs/snapshot
[list of 1797 snapshots here]
real    0m0.023s
user    0m0.008s
sys     0m0.014s

# time zfs list -r -t snapshot -H -o name vz/subvol-300-disk-1
[same list of 1797 snapshots]
real    1m23.092s
user    0m0.110s
sys     0m0.758s

Is this bug specific to ZFS-on-Linux?

Can anybody with a Solaris or FreeBSD ZFS box perform a similar test (on a filesystem with hundreds of snapshots on spinning hard disks)?

Is there a workaround to get a quick list of snapshots for a volume, which by its nature does not have a .zfs directory?

I've run the above test with ZFS-on-Linux 0.6.5.2-2-wheezy on kernel 2.6.32-43-pve x86_64 (Proxmox) but I've always seen this issue, both on older and newer ZFS and kernel versions.

Here are the pool stats:

# zpool list
NAME   SIZE  ALLOC   FREE  EXPANDSZ   FRAG    CAP  DEDUP  HEALTH  ALTROOT
vz    25.2T  9.42T  15.8T         -     5%    37%  1.00x  ONLINE  -

It contains 114 filesystems and 1 volume, each with hundreds of snapshots, as this is a zfs send / zfs recv backup server.

Solution: zfs list is slow because it fetches additional information, even if it's not displayed. The solution is adding both -o name -s name, that is, using zfs list -t snapshot -o name -s name

zfs zfsonlinux

2 Answers

  1. zfs list -t snapshot always takes many orders of magnitude more time to run than ls .zfs/snapshot

    You're also comparing two completely different operations.

    zfs list -t snapshot enumerates all the ZFS snapshots on the system - and provides a lot of information about those snapshots, such as the amount of space used. Run that under strace to see the system calls made.

    ls .zfs/snapshot is just emitting a simple name list from a directory. There's nothing to do other than read the names - and provide nothing else.

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  2. Best Answer

    Snapshot operations are a function of the number of snapshots you have, RAM, disk performance and drive space. This would be a general ZFS issue, not something unique to the Linux variant.

    The better question is: Why you have 1797 snapshots of a zvol? This is definitely more than recommended and makes me wonder what else is happening on the system.

    People say "ZFS snapshots are free", but that's not always true.

    While ZFS snaps don't have an impact on production performance, the high number you have clearly require disk accesses to enumerate.

    Disk access time > RAM access time, hence the order of magnitude difference.

    strace output. Note the time per syscall and imagine how poorly it would scale with the number of snapshots in your filesystem.

    # strace -c ls /ppro/.zfs/snapshot

% time     seconds  usecs/call     calls    errors syscall
------ ----------- ----------- --------- --------- ----------------
  0.00    0.000000           0        10           read
  0.00    0.000000           0        17           write
  0.00    0.000000           0        12           open
  0.00    0.000000           0        14           close
  0.00    0.000000           0         1           stat
  0.00    0.000000           0        12           fstat
  0.00    0.000000           0        28           mmap
  0.00    0.000000           0        16           mprotect
  0.00    0.000000           0         3           munmap
  0.00    0.000000           0         3           brk
  0.00    0.000000           0         2           rt_sigaction
  0.00    0.000000           0         1           rt_sigprocmask
  0.00    0.000000           0         2           ioctl
  0.00    0.000000           0         1         1 access
  0.00    0.000000           0         1           execve
  0.00    0.000000           0         1           fcntl
  0.00    0.000000           0         2           getdents
  0.00    0.000000           0         1           getrlimit
  0.00    0.000000           0         1           statfs
  0.00    0.000000           0         1           arch_prctl
  0.00    0.000000           0         2         1 futex
  0.00    0.000000           0         1           set_tid_address
  0.00    0.000000           0         1           set_robust_list
------ ----------- ----------- --------- --------- ----------------
100.00    0.000000                   133         2 total

    versus

    # strace -c zfs list -t snapshot

% time     seconds  usecs/call     calls    errors syscall
------ ----------- ----------- --------- --------- ----------------
100.00    0.003637          60        61         7 ioctl
  0.00    0.000000           0        12           read
  0.00    0.000000           0        50           write
  0.00    0.000000           0        19           open
  0.00    0.000000           0        19           close
  0.00    0.000000           0        15           fstat
  0.00    0.000000           0        37           mmap
  0.00    0.000000           0        19           mprotect
  0.00    0.000000           0         1           munmap
  0.00    0.000000           0         4           brk
  0.00    0.000000           0         2           rt_sigaction
  0.00    0.000000           0         1           rt_sigprocmask
  0.00    0.000000           0         3         1 access
  0.00    0.000000           0         1           execve
  0.00    0.000000           0         1           getrlimit
  0.00    0.000000           0         1           arch_prctl
  0.00    0.000000           0         2         1 futex
  0.00    0.000000           0         1           set_tid_address
  0.00    0.000000           0         1           set_robust_list
------ ----------- ----------- --------- --------- ----------------
100.00    0.003637                   250         9 total
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