4 ms·

There is an elephant in the room. btrfs filesystem defrag -r / You can't do that with ZFS. People can really hurt you with that. https://www.usenix.org/sy

by chasil 5y ago

There is an elephant in the room. btrfs filesystem defrag -r / You can't do that with ZFS. People can really hurt you with that. https://www.usenix.org/system/files/login/articles/login_summer17_02_conway.pdf https://www.usenix.org/system/files/login/articles/login_sum...

pmarreck 5y ago
"Git aging workload on btrfs on HDD. The overall slowdown is 20.6x." Well, this article is fascinating, but maybe the reason why btrfs has built-in defrag functionality is because without it, performance degrades incredibly steeply?
- pmarreck 5y ago
  Followup to this: I've had a btrfs defrag running for over 12 hours now :O
  - kasabali 5y ago
    Are you on Linux 5.16? You might've hit the infinite defrag bug.
    - pmarreck 5y ago
      i'm on arch, so probably. any info on this?
      - kasabali 5y ago
        https://www.reddit.com/r/btrfs/comments/sh0otq https://www.reddit.com/r/btrfs/comments/sh0otq
        pmarreck 5y ago
        interesting, my btrfs mount in fstab did not have autodefrag on, but I also have it in a VMWare VM, so it is probably that
infogulch 5y ago
Interesting. That was specifically tested on ZFS in a prior paper referenced by that one and published in 2017: https://www.usenix.org/system/files/conference/fast17/fast17-conway.pdf https://www.usenix.org/system/files/conference/fast17/fast17... Glancing through, it seems ZFS is about middle of the pack of the analyzed filesystems (BetrFS Btrfs ext4 F2FS XFS ZFS) in terms of performance aging caused by change history. And only BetrFS did not show any aging from these tests and was surprisingly stable throughout. I'm curious if that's changed in recent versions of any the analyzed file systems, I know ZFS has had a major release recently.
- benjamir 5y ago
  When a comment is worth more than the discussed random blog entry... Thanks for the USENIX link!
Dylan16807 5y ago
I can't usually do that on Btrfs either. Whenever it defragments something that's COW, it makes a new copy with 1 use, and leaves all the other snapshots/reflinks pointing at the old copy.
- the8472 5y ago
  You can defrag and then run an offline dedup to restore CoW.
  - Dylan16807 5y ago
    Can I be reasonably sure that the dedup will pick the defragmented version?
    - the8472 5y ago
      Well, most of dedup runs in userspace. But yes, the existing tools are good enough at it to work across snapshots.
curt15 5y ago
ZFS has a `recordsize` parameter that can mitigate fragmentation if tuned correctly for the workload. This allows zfs to perform pretty competitively on workloads that would cripple btrfs unless one disabled copy-on-write (and therefore checksumming and compression) altogether [1]. [1] https://www.percona.com/blog/mysql-zfs-performance-update/ https://www.percona.com/blog/mysql-zfs-performance-update/

by chasil 5y ago

pmarreck 5y ago

"Git aging workload on btrfs on HDD. The overall slowdown is 20.6x." Well, this article is fascinating, but maybe the reason why btrfs has built-in defrag functionality is because without it, performance degrades incredibly steeply?

pmarreck 5y ago
Followup to this: I've had a btrfs defrag running for over 12 hours now :O
- kasabali 5y ago
  Are you on Linux 5.16? You might've hit the infinite defrag bug.
  - pmarreck 5y ago
    i'm on arch, so probably. any info on this?
    - kasabali 5y ago
      https://www.reddit.com/r/btrfs/comments/sh0otq https://www.reddit.com/r/btrfs/comments/sh0otq
      - pmarreck 5y ago
        interesting, my btrfs mount in fstab did not have autodefrag on, but I also have it in a VMWare VM, so it is probably that

infogulch 5y ago

Interesting. That was specifically tested on ZFS in a prior paper referenced by that one and published in 2017: https://www.usenix.org/system/files/conference/fast17/fast17-conway.pdf https://www.usenix.org/system/files/conference/fast17/fast17... Glancing through, it seems ZFS is about middle of the pack of the analyzed filesystems (BetrFS Btrfs ext4 F2FS XFS ZFS) in terms of performance aging caused by change history. And only BetrFS did not show any aging from these tests and was surprisingly stable throughout. I'm curious if that's changed in recent versions of any the analyzed file systems, I know ZFS has had a major release recently.

benjamir 5y ago
When a comment is worth more than the discussed random blog entry... Thanks for the USENIX link!

Dylan16807 5y ago

I can't usually do that on Btrfs either. Whenever it defragments something that's COW, it makes a new copy with 1 use, and leaves all the other snapshots/reflinks pointing at the old copy.

the8472 5y ago
You can defrag and then run an offline dedup to restore CoW.
- Dylan16807 5y ago
  Can I be reasonably sure that the dedup will pick the defragmented version?
  - the8472 5y ago
    Well, most of dedup runs in userspace. But yes, the existing tools are good enough at it to work across snapshots.

curt15 5y ago

ZFS has a `recordsize` parameter that can mitigate fragmentation if tuned correctly for the workload. This allows zfs to perform pretty competitively on workloads that would cripple btrfs unless one disabled copy-on-write (and therefore checksumming and compression) altogether [1]. [1] https://www.percona.com/blog/mysql-zfs-performance-update/ https://www.percona.com/blog/mysql-zfs-performance-update/