Will BTRFS repair or restore data if corrupted?

Will BTRFS repair or restore data if corrupted?

[Waxhead]

Hi,

 From what I have read BTRFS does replace a bad copy of data with a 
known good copy (if it has one). Will BTRFS try to repair the corrupt 
data or will it simply silently restore the data without the user 
knowing that a file has been "fixed"?

Re: Will BTRFS repair or restore data if corrupted?

[Chester]

On Wed, Jan 25, 2012 at 5:27 PM, Waxhead <waxhead@online.no> wrote:
> Hi,
>
> From what I have read BTRFS does replace a bad copy of data with a kn=
own
> good copy (if it has one). Will BTRFS try to repair the corrupt data =
or will
> it simply silently restore the data without the user knowing that a f=
ile has
> been "fixed"?
=46rom what I've seen, it'll quietly restore the data without the user
knowing. The only indication would be in dmesg where there should be a
message about incorrect checksums and btrfs fixing it automatically.
>
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Re: Will BTRFS repair or restore data if corrupted?

[Hugo Mills]

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On Thu, Jan 26, 2012 at 12:27:57AM +0100, Waxhead wrote:
> Hi,
> 
> From what I have read BTRFS does replace a bad copy of data with a
> known good copy (if it has one).

   Correct.

> Will BTRFS try to repair the corrupt data or will it simply silently
> restore the data without the user knowing that a file has been
> "fixed"?

   No, it'll just return the good copy and report the failure in the
system logs. If you want to fix the corrupt data, you need to use
scrub, which will check everything and fix blocks with failed
checksums.

   Hugo.

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Re: Will BTRFS repair or restore data if corrupted?

[Stefan Behrens]

On 1/26/2012 9:59 AM, Hugo Mills wrote:
> On Thu, Jan 26, 2012 at 12:27:57AM +0100, Waxhead wrote:
[...]
>> Will BTRFS try to repair the corrupt data or will it simply silently
>> restore the data without the user knowing that a file has been
>> "fixed"?
> 
>    No, it'll just return the good copy and report the failure in the
> system logs. If you want to fix the corrupt data, you need to use
> scrub, which will check everything and fix blocks with failed
> checksums.

Since 3.2, btrfs rewrites the corrupt disk block (commit 4a54c8c and
f4a8e65 from Jan Schmidt), even without scrub.

Re: Will BTRFS repair or restore data if corrupted?

[Zoiled]

Stefan Behrens wrote:
> On 1/26/2012 9:59 AM, Hugo Mills wrote:
>> On Thu, Jan 26, 2012 at 12:27:57AM +0100, Waxhead wrote:
> [...]
>>> Will BTRFS try to repair the corrupt data or will it simply silently
>>> restore the data without the user knowing that a file has been
>>> "fixed"?
>>     No, it'll just return the good copy and report the failure in the
>> system logs. If you want to fix the corrupt data, you need to use
>> scrub, which will check everything and fix blocks with failed
>> checksums.
> Since 3.2, btrfs rewrites the corrupt disk block (commit 4a54c8c and
> f4a8e65 from Jan Schmidt), even without scrub.
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>
>
>
So if I for example edit a text file three times and store it I can get 
the following.
Version1: I currently like cheese
Version2: I currently like onions
Version3: I currently like apples
As far as I understand a disk corruption might result in me suddenly 
liking onions (or even cheese) instead of apples without any warning 
except in syslog.?! I really hope I have misunderstood the concept and 
that there is some error correction codes somewhere.

Re: Will BTRFS repair or restore data if corrupted?

[cwillu]

> So if I for example edit a text file three times and store it I can get the
> following.
> Version1: I currently like cheese
> Version2: I currently like onions
> Version3: I currently like apples
> As far as I understand a disk corruption might result in me suddenly liking
> onions (or even cheese) instead of apples without any warning except in
> syslog.?! I really hope I have misunderstood the concept and that there is
> some error correction codes somewhere.

Yes, you've completely misunderstood the concept :p

There are crc's on each 4k block of data; if one copy fails the
checksum, and a second copy is available, and that copy does match,
then the good data will be returned and btrfs will overwrite the
corrupted copy with the good copy.  If there isn't another copy, then
an io error will be returned instead.

Re: Will BTRFS repair or restore data if corrupted?

[Zoiled]

cwillu wrote:
>> So if I for example edit a text file three times and store it I can get the
>> following.
>> Version1: I currently like cheese
>> Version2: I currently like onions
>> Version3: I currently like apples
>> As far as I understand a disk corruption might result in me suddenly liking
>> onions (or even cheese) instead of apples without any warning except in
>> syslog.?! I really hope I have misunderstood the concept and that there is
>> some error correction codes somewhere.
> Yes, you've completely misunderstood the concept :p
>
> There are crc's on each 4k block of data; if one copy fails the
> checksum, and a second copy is available, and that copy does match,
> then the good data will be returned and btrfs will overwrite the
> corrupted copy with the good copy.  If there isn't another copy, then
> an io error will be returned instead.
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>
>
>
Phew... that sounds better :)