Re: question to btrfs scrub

[Qu Wenruo <quwenruo.btrfs@gmx.com>]

On 2023/7/8 04:48, Bernd Lentes wrote:
>
>> -----Original Message-----
>> From: Qu Wenruo <quwenruo.btrfs@gmx.com>
>> Sent: Thursday, July 6, 2023 8:20 AM
>> To: Bernd Lentes <bernd.lentes@helmholtz-muenchen.de>; Qu Wenruo
>> <wqu@suse.com>; linux-btrfs <linux-btrfs@vger.kernel.org>
>> Subject: Re: question to btrfs scrub
>
>
>
>> Checksum have two functions:
>>
>> - Detect errors
>>    As long as it's not a false alert, then it makes sense.
>>    But please keep in mind that, a csum mismatch will prevent anyone
>>    from reading that corrupted data, whether this is the expected
>>    behavior really depends on your workload.
>>
>>    E.g. if some archive with built-in error detect and recovery (like RAR
>>    files), it's definitely not a good idea to return -EIO for the whole
>>    block, other than reading out the corrupted data and let the software
>>    to handle them.
>>
>> - Recover the good data from the extra mirrors
>>    This only works if you're using profiles with duplication (DUP,
>>    RAID1*, RAID10, RAID5, RAID6).
>>    Otherwise btrfs won't be able to recover anything.
>>
>> In your case, since you're already using LVM, thus I believe the fs is using
>> default profiles (DUP for meta, SINGLE for data), thus there would be no extra
>> copy to recover from.
>>
>> So there is really error detection functionality lost if go nodatasum.
>>
>>> OK. I know which VM images produced checksum errors. I delete them and
>> restore them from the backup.
>>
>> You mentioned snapshots are utilized for those images, thus you have to
>> delete all the involved files, including ones in the snapshots.
>>
>>> So it finds errors in the data csum, correct ?
>>
>> Yes, either there are some files not deleted, or the file is snapshoted.
>
> I don't understand what you mean with that.

Is the file you deleted also in other snapshots?

And are you sure you deleted every file reported in dmesg by scrub?

Another possibility is, scrub reports can be ratelimited, thus some
files are not properly prompted.

Thanks,
Qu

>
> Thanks.
>
> Bernd
>
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