Re: Question about resync in RAID5

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

On 16-04-13 11:59, Robin Dong wrote:
> Dear Raid experts,
>
> I have a soft RAID5 volume and after one disk failed I replaced it with
> a new hard-disk. Then the raid5 volume begin to resync with WHOLE new
> disk.
> There is only 1G data in the RAID5 volume so I think resync whole disk
> is not efficient.
The md driver however does not know this. It syncs everything.

Having said that, how do you know there's only 1 GB in use? Maybe there 
is a hidden partition for the remainder of the disk? Or, maybe it houses 
a encrypted container and in that container a nother encrpyted 
countainer (check truecrypt on deniability).

Fact is, you do not know how much data is or is not used, well not for sure.
> Take ZFS for example, when replacing with a new disk, it only resync the
> data which are written after the creation of the volume.
but md is only the raid layer, zfs is the lower layer AND the 
filesystem, you can't really fairly compare them. Btrfs may do this, but 
I don't know and don't think it's ready as replacement yet anyway.
>
> Is there any method to just resync WRITTEN data to new-added-disk ? Or
> any developing plan to add this feature?
I highly doubt it, since, as said above, the md layer could not ever 
possibly know what data is on the disk.
>
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