Is this normal? Should I use scrub?

Hello,

I have a 6 device RAID-1 filesystem:

$ sudo btrfs fi df /srv/tank
Data, RAID1: total=1.24TiB, used=1.24TiB
System, RAID1: total=32.00MiB, used=184.00KiB
Metadata, RAID1: total=3.00GiB, used=1.65GiB
unknown, single: total=512.00MiB, used=0.00
$ sudo btrfs fi sh /srv/tank
Label: 'tank'  uuid: 472ee2b3-4dc3-4fc1-80bc-5ba967069ceb
        Total devices 6 FS bytes used 1.24TiB
        devid    2 size 1.82TiB used 384.03GiB path /dev/sdh
        devid    3 size 1.82TiB used 383.00GiB path /dev/sdg
        devid    4 size 1.82TiB used 384.00GiB path /dev/sdf
        devid    5 size 2.73TiB used 1.13TiB path /dev/sdk
        devid    6 size 1.82TiB used 121.00GiB path /dev/sdj
        devid    7 size 2.73TiB used 116.00GiB path /dev/sde

Btrfs v3.14.2

All of these devices are in an external eSATA enclosure.

A few days ago (I believe) something went wrong with the enclosure hardware and the SCSI bus kept getting reset over and over. At one point three of the six devices were kicked out and the filesystem was left running (read-only) on three devices.

Through some trial and error I determined that the enclosure was taking exception to one of the devices, and by removing it I was able to get things up and running with five devices, writeable, mounted in degraded mode. /dev/sdk is the device that was kept out of the filesystem.

I do not believe that there is anything wrong with /dev/sdk as I put it in another system and was able to read it entirely, do SMART long tests on it, etc.

I wasn't able to prove it is a hardware problem until I took the enclosure out of service as it's the only enclosure I had. So that's a task for later.

I have now got a new enclosure and put this system back together with all six devices. I was not expecting this filesystem to mount without assistance on boot because of /dev/sdk being "stale" compared to the other devices. I suppose this incorrect view is a holdover from my experience with mdadm.

Anyway, I booted it and /srv/tank was mounted automatically with all six devices. I got a bunch of these messages as soon as it was mounted:

http://pastie.org/private/2ghahjwtzlcm6hwp66hkg

There's lots more of it but it's all like that. That paste is from the end of the log and there haven't been any more such message since, so that's about 20 minutes (the times are in GMT).

Is that normal output indicating that btrfs is repairing the "staleness" of sdk from the other copy?

I seem to be able to use the filesystem and a cursory inspection isn't turning up anything that I can't read or that seems corrupted. I will now run checksums against my last good backup.

Should I run a scrub as well?

Cheers,
Andy

Hi, Andy,

[snip tale of a filesystem with out of data data on one copy of the RAID]

Yes, exactly. That output you pasted looks pretty much exactly like
what I'd expect to see in the situation described above. You might also expect to see some checksum errors corrected in the data, as well as the metadata messages you're getting.

Yes. The output you've had so far will be just the pieces that the
FS has tried to read, and where, as a result, it's been able to detect the out-of-date data. A scrub will check and fix everything.

Hugo.

Hugo Mills             | My karma has run over my dogma.
hugo@... carfax.org.uk |
http://carfax.org.uk/  |
PGP: 65E74AC0          |

Hi Hugo,

Thanks for your help.

Thanks, things seem to be fine now. :)

What's the difference between "verufy" and "csum" here?

scrub status for 472ee2b3-4dc3-4fc1-80bc-5ba967069ceb
scrub device /dev/sdh (id 2) history
scrub started at Wed Apr 1 20:05:58 2015 and finished after 14642 seconds total bytes scrubbed: 383.42GiB with 0 errors
scrub device /dev/sdg (id 3) history
scrub started at Wed Apr 1 20:05:58 2015 and finished after 14504 seconds total bytes scrubbed: 382.62GiB with 0 errors
scrub device /dev/sdf (id 4) history
scrub started at Wed Apr 1 20:05:58 2015 and finished after 14436 seconds total bytes scrubbed: 383.00GiB with 0 errors
scrub device /dev/sdk (id 5) history
scrub started at Wed Apr 1 20:05:58 2015 and finished after 21156 seconds total bytes scrubbed: 1.13TiB with 14530 errors
error details: verify=10909 csum=3621
corrected errors: 14530, uncorrectable errors: 0, unverified errors: 0
scrub device /dev/sdj (id 6) history
scrub started at Wed Apr 1 20:05:58 2015 and finished after 5693 seconds total bytes scrubbed: 119.42GiB with 0 errors
scrub device /dev/sde (id 7) history
scrub started at Wed Apr 1 20:05:58 2015 and finished after 5282 seconds total bytes scrubbed: 114.45GiB with 0 errors

Cheers,
Andy

Makes a change from you answering my questions. :)

verify would be where the internal consistency checks for metadata
failed. That might be, for example, where it's detected that a tree node has a newer transaction ID (effectively a monotonic timestamp) than its parent. This should never happen, so the parent is probably out of date. If there's another copy of the metadata that doesn't have the same problem, it can be used to repair the obviously-wrong copy.

csum is where the checksum validation failed -- this would be, for
example, where some data was modified on one copy and left unchanged on the older copy, but the metadata for both copies was updated. In that case, the data on the out-of-date drive wouldn't match the checksum, and needs to be updated from the good copy.

Hugo.

Hugo Mills             | Debugging is like hitting yourself in the head with
hugo@... carfax.org.uk | hammer: it feels so good when you find the bug, and
http://carfax.org.uk/  | you're allowed to stop debugging.
PGP: 65E74AC0          |                                        PotatoEngineer