Re: RAID5 reconstruction ?

[Goswin von Brederlow <goswin-v-b@web.de>]

Redeeman <redeeman@metanurb.dk> writes:

> On Sat, 2009-05-30 at 14:35 +0100, John Robinson wrote:
>> On 30/05/2009 06:44, SandeepKsinha wrote:
>> > Hi all,
>> > 
>> > Say If I have a RAID 5 array of 50GB of five disks of 10GB each.
>> > 
>> > I have data of 5GB. When a disk fails and replaced with a spare disk.
>> > Will the reconstruction happen only for the 5GB allocated disk blocks
>> > or it will happen for the whole disk size.
>> 
>> The whole disc size, for now anyway; md does not currently note which 
>> blocks have been used by its client (the filesystem, LVM, whatever).
>> 
>> > Is it possible to make  reconstruction intelligent enough to keep it optimized ?
>> 
>> This has been discussed in combination with supporting SSD drives' TRIM 
>> function, and would mean md had to keep track of used chunks or possibly 
>> even sectors using a bitmap or something like that, but whether anyone's 
>> working on it I don't know.
>
> I would say it should be possible to 'query' the filesystem for that
> information. Obviously this will only work if you run a filesystem on it
> which supports it, but it would seem like a nicer solution than a bitmap
> for it.

On the other hand checking a bitmap is quick. You could use the bitmap
not only for reconstruction but also for reads. If the bitmap say the
block is unused you can skip the read and just zero fill the
buffer. This would speed up reads and writes that don't cover the full
stripe. And compared to a raid with current bitmap there shouldn't be
any real slowdown for the extra "unused" bit.

The only special case for resync would be that if all data blocks in a
stripe are unused then the parity blocks can be marked unused too.

MfG
        Goswin