Re: md: persistent (file-backed) bitmap

[Paul Clements <paul.clements@steeleye.com>]

Hi Neil,

Neil Brown wrote:

>  I would like to change the "RESYNC_MASK" bit to mean:
>    At least one block in this chunk is out-of-sync.

OK

>  Then:
>   - When we read the bitmap from disk we set this bit and the
>     SHADOW_MASK, but leave the counter at zero.
>   - When we get a failed write, we set this bit, but still decrement
>     the counter. 

>   - When we are performing a resync, we periodically clear the bit on
>     recently completed chunks.

I assume that this also means that the counter will get incremented 
before each read-for-resync is submitted. Perhaps you've already 
considered that?


>   - We only clear the SHADOW_MASK and on-disk bit when the counter
>     hits zero *and* this bit is clear.
> 
>  I would find this approach a lot easier to understand.  Are you OK
>  with it?

It sounds reasonable. Probably a little simpler too...

>  Also, I would like to move the bitmap_testbit in md_do_sync down into
>  the personality.  This should make life easier for other
>  personalities like raid10 which use a very different approach for
>  resync than for recovery.

OK, that sounds fine.

--
Paul