Re: Recovering from csum errors

[Hugo Mills <hugo@carfax.org.uk>]

[-- Attachment #1: Type: text/plain, Size: 2175 bytes --]

On Mon, Sep 02, 2013 at 11:41:12PM +0200, Rain Maker wrote:
> Hello list,
> 
> So, I ran a full scrub, and, luckily, it only found 6 csum errors
> (these 6). The damage therefore seems to be contained in "just" 1
> file.
> 
> Now, I removed the offending file. But is there something else I
> should have done to recover the data in this file? Can it be
> recovered?

   No, and no. The data's failing a checksum, so it's basically
broken. If you had a btrfs RAID-1 configuration, the FS would be able
to recover from one broken copy using the other (good) copy.

> I'm running 3.11-rc7. It is a single disk btrfs filesystem. I have
> several subvolumes defined, one of which for VMWare Workstation (on
> which the corruption took place).

   Aaah, the VM workload could explain this. There's some (known,
won't-fix) issues with (I think) direct-IO in VM guests that can cause
bad checksums to be written under some circumstances.

   I'm not 100% certain, but I _think_ that making your VM images
nocow (create an empty file with touch; use chattr +C; extend the file
to the right size) may help prevent these problems.

> I checked the SMART values, they all seem OK. The harddisks in this
> machine are less then a month old. I replaced them after seeing
> similar messages on the "old" disks.
> 
> Is the only logical explanation for this some kind of hardware failure
> (SATA controller, power supply...), or could there be something more
> to this?

   As above, there's some direct-IO problems with data changing
in-flight that can lead to bad checksums. Fixing the issue would cause
some fairly serious slow-downs in performance for that case, which is
rather against what direct-IO is trying to do, so I think it's
unlikely the behaviour will be changed.

   Of course, I could be completely wrong about all this, and you've
got bad RAM or PSU something...

   Hugo.

-- 
=== Hugo Mills: hugo@... carfax.org.uk | darksatanic.net | lug.org.uk ===
  PGP key: 65E74AC0 from wwwkeys.eu.pgp.net or http://www.carfax.org.uk
    --- "What are we going to do tonight?" "The same thing we do ---     
            every night, Pinky.  Try to take over the world!"            

[-- Attachment #2: Digital signature --]
[-- Type: application/pgp-signature, Size: 828 bytes --]