Re: [RFC 0/3] Btrfs checksum offload

[Kanchan Joshi <joshi.k@samsung.com>]

On 1/31/2025 3:59 PM, Johannes Thumshirn wrote:
>> I tested the series for read, but only the success cases. In this case
>> checksum generation/verification happens only within the device. It was
>> slightly tricky to inject an error and I skipped that.
>>
>> Since separate checksum I/Os are omitted, this is about handling the
>> error condition in data read I/O path itself. I have not yet checked if
>> repair code triggers when Btrfs is working with existing 'nodatasum'
>> mount option. But I get your point that this needs to be handled.
>>
> So this as of now disables one of the most useful features of the FS,
> repairing bad data. The whole "story" for the RAID code in the FS is
> build around this assumption, that we can repair bad data, unlike say MD
> RAID.

Does repairing-bad-data work when Btrfs is mounted with NODATASUM?
If not, should not the proposed option be seen as an upgrade over that?

You might be knowing, but I do not know how does Btrfs currently decide 
to apply NODATSUM! With these patches it becomes possible to know if 
checksum-offload is supported by the underlying hardware. And that makes 
it possible to apply NODATASUM in an informed manner.

I have not reduced anything, but added an opt-in for deployments that 
may have a different definition of what is useful. Not all planets are 
Mars. The cost of checksum tree will be different (say on QLC vs SLC).