Re: [PATCH 1/2] md bitmap bug fixes

On Monday March 14, lmb@suse.de wrote:

On 2005-03-14T15:43:52, Neil Brown [off-list ref] wrote:

Hi there, just a question about how the bitmap stuff works with
1++-redundancy, say RAID1 with 2 mirrors, or RAID6.

I assume you mean RAID1 with 3 drives (there isn't really one main
drive and all the others are mirrors - all drives are nearly equal).

We haven't put any significant work into bitmap intent logging for
levels other than raid1, so some of the answer may be pure theory.

One disk fails and is replaced/reattached, and resync begins. Now
another disk fails and is replaced. Is the bitmap local to each
disk?

Bitmap is for the whole array, not per-disk.
If there are any failed drives, bits are not cleared from the bitmap.
If a drive fails then any active resync is aborted and restarted
(possibly this is not optimal...).

If a failed disk is re-attached, then only the blocks changed since
that the array was known-good are resynced.  If a new drive is added, all
blocks are synced.

And in case of RAID1, with 4 disks (and two of them resyncing), could
disk3 be rebuild from disk1 and disk4 from disk2 (as to optimize disk
bandwidth)?

If two are resyncing, they will resync in step with each other so
there is no point in reading from disk2 as well as disk1.  Just read
from disk 1 and write to disks 3 and 4.

Does that answer your questions?

NeilBrown