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.
>
>> 
>> Cheers,
>> 
>> John.

And just when I hit send I thought of something else.

Instead of the initial sync when creating a raid the bitmap could just
mark all blocks as unused. Much faster raid creation.

MfG
        Goswin