RAIDZ3 "chunk" sound every 5 seconds

[loneboat]

I recently set up a new node for my cluster. On previous installs I have done mirrored vdevs, which I understand isn't very space-efficient, but has good redundancy. However in this most recent case I had a stack of spare 3TB drives, so I used RAIDZ3 to get a decent tradeoff for redundancy/space. This is my first time using RAIDZ3.

I have noticed that every 5 seconds all 8 of the drives make a "chunk" sound in sync with each other (see video below, kinda hard to make out the sound without headphones). This seems fairly consistently at 5-second intervals, and happens whether the server is under load or not.

Any idea what is causing this? I haven't noticed this behavior in my other installs, but that may be because I've been using a different array type.

Here is the output from zpool status:

~# zpool status
  pool: rpool
state: ONLINE
status: Some supported features are not enabled on the pool. The pool can
        still be used, but some features are unavailable.
action: Enable all features using 'zpool upgrade'. Once this is done,
        the pool may no longer be accessible by software that does not support
        the features. See zpool-features(5) for details.
config:

        NAME                              STATE     READ WRITE CKSUM
        rpool                             ONLINE       0     0     0
          raidz3-0                        ONLINE       0     0     0
            scsi-35000cca0580cc7c8-part3  ONLINE       0     0     0
            scsi-350000c0f02f00d60-part3  ONLINE       0     0     0
            scsi-350000c0f0299a030-part3  ONLINE       0     0     0
            scsi-35000cca058a8f1bc-part3  ONLINE       0     0     0
            scsi-350000c0f045a8e54-part3  ONLINE       0     0     0
            scsi-35000cca0585beef8-part3  ONLINE       0     0     0
            scsi-350000c0f042fd378-part3  ONLINE       0     0     0
            scsi-35000cca0587a9c44-part3  ONLINE       0     0     0

errors: No known data errors

 

[H4R0]

Zfs caches async writes and syncs them every 5 seconds, it's the zfs_txg_timeout option.

 

[loneboat]

Zfs caches async writes and syncs them every 5 seconds, it's the zfs_txg_timeout option.

Aah that explains it perfectly - thank you for your help!