Re: [RFE] Please, add optional RAID1 feature (= chunk checksums) to make it more robust

[Oliver Schinagl <oliver+list@schinagl.nl>]

On 20-07-12 13:07, Jaromir Capik wrote:
>> This is a very invasive change that you ask, conceptually,
>> man-hours-wise, performance-wise, ondisk-format wise, space-wise
> Yes ... I'm aware of possibly high number of man-hours.
> If we talk about space ... 0.78% is not so invasive, is it?
> On-disk format ... interleaving chunks with checksum sectors doesn't
> seem to me a complicated math ...
>
>    chunk_starting_sector = chunk_number * (chunk_size_in_sectors + 1)
>
> ... of course this is relative to the chunk area offset.
>
>> also it really should stay at another layer, preferably below the
>> RAID
> but how would you like to implement that if the lower level is known
> to be unreliable enough?
>
>> (btrfs and zfs do this above though).
> Btrfs and zfs has it's own RAID layer, so there's no need for
> underlying MD-RAID. But I haven't studied how exactly it's done
> there.
>
>> This should probably be a
>> DM/LVM
>> project.
> LVM ? How do you want to implement that in LVM? You would create
> two big PVs with two big logical partitions protected by checksums?
> The mdraid layer would be built on top of these, right?
> That could possibly work too if LVM returns read errors for blocks
> with incorrect checksums. I'm not fully against that idea.
>
>> Drives do this already, they have checksums (google for
>> reed-solomon).
>> If the checksums are not long enough you should use different drives.
>> But in my life I never saw a "silent data corruption" like the one
>> you say.
> I believe I've mentioned my experience with such nasty HDD behaviour
> in my previous email. I also don't like that, but it apparently
> happens and we can't rely on the proper hardware functioning
> especially when it's unreliable by nature.
Actually, I've had quite some dataloss due to a 
hardrive/controller/cabling not working properly (no clue what caused 
it) but raid5 never complained. To this date, I do not know what 
happened and why my data was corrupt.
> Regards,
> Jaromir.
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