What does scrub do?

[Alex <alex@bpmit.com>]

Hi all,

Debian testing/Jessie-to-be; except kernels/btrfs-tools are from unstable so
usually couple of weeks later than you/Linus publish.
Linux XX 3.13-1-amd64 #1 SMP Debian 3.13.7-1 (2014-03-25) x86_64
Btrfs-tools v3.12 Debian standard (not particularly messed with looks like) 

I've never had scrub report anything other than 0 (zero) errors. Ever.
Yet I've had more than one ( ;-) ) problem which required btrfs-zero-log
and/or btrfs --repair. This are usually my fault - fixed it 'til it broke.

root@XX ~ # btrfs scrub status /
scrub status for f8152a67-3c2e-4da1-812e-9a6ab2ad1102
scrub started at Fri Apr 11 09:55:36 2014 and finished after 44 seconds
total bytes scrubbed: 1.40GiB with 0 errors

[    7.502338] btrfs: device label china devid 1 transid 938773 /dev/vda1
[    7.514213] btrfs: device label china devid 1 transid 938773 /dev/vda1
[    7.530893] btrfs: disk space caching is enabled
[    7.530897] btrfs: has skinny extents
[    7.720288] btrfs: bdev /dev/vda1 errs: wr 0, rd 0, flush 0, corrupt 66,
gen 2
[   18.967319] btrfs: device label china devid 1 transid 938773 /dev/vda1
[   19.360767] btrfs: device label china devid 1 transid 938773 /dev/vda1

This scrub and dmesg were taken within minutes of each other. So what it the
utility of running scrub? Or have I got the the wrong idea of what scrub
should report. This VM guest doesn't get messed with often, and is kept 

Very small KVM virtual machine - easy to send you a btrfs dump. Almost
vanilla set-up too. Just say the word.

Have been running btrfs here for quite some while (years, since Linux3.1 I
think) on server. Very very stable (lzo compression sometimes not quite as
stable as zlib and I only run it on the desktop m/c).

People: for auto snapshots use Snapper (a la SUSE) which is now in Debian et
al. Only peculiarity is that clear-down of daily snapshots only happens in
the night so you don't need to put many/any hourly snapshots in.

Thank you. And well done/thank you to the contributors.

Al.

PS: please get the 3.14 tools release out - perhaps the fixes have 
already gone through the tree and I am just shouting at the wind.