Re: btrfs csum failed

[Martin Schitter <ms@mur.at>]

Am 2011-05-04 15:23, schrieb Edward Ned Harvey:
>> From: linux-btrfs-owner@vger.kernel.org [mailto:linux-btrfs-
>> owner@vger.kernel.org] On Behalf Of Martin Schitter
>>
>> well -- i am doing a backup of all images every night. this
>> process should work like a simple "scrub" because all data (and its
>> checksumes) will be read.
>
> Sorry, not correct.  When you read all the data using something in
> user-land, the OS only needs to read one side of the data.  It can
> accelerate by staggering the read requests across multiple disks.  So
> some sectors remain unread on some disks.
>
> When you scrub, it reads all the data from all the redundant copies
> (mirrored or raid) on all the individual disks in the raid set.

ok -- i see -- you're right!

i know, there a some befits in the way btrfs and zfs implement RAID / 
multiply disk usage and checksumming, but i a also want to stay on the 
save side, when it comes to real practical problems. so i decided to use 
'classical' linux software RAID-1 as the base layer. that's a very old 
fashioned solution, but it usually simply works... and you can change a 
broken disk without any respect of the used filesystem(s). in general i 
try to use btrfs only on account of its snapshot features in a very 
simple way.

it looks very strange to me, that i don't see any SMART warnings on the 
harddisks or errors on other filsystems on the same raid-array. there 
was also no reboot, power-failure or similar when the corruption 
suddenly appeared. so i think, a btrfs bug would be the most evident 
explanation.

martin