On 4/18/12 4:26 AM, NeilBrown wrote:
On Tue, 17 Apr 2012 07:46:03 -0700 Dan Williams<dan.j.williams@xxxxxxxxx>
wrote:
On Tue, Apr 17, 2012 at 1:35 AM, Shaohua Li<shli@xxxxxxxxxx> wrote:
Discard for raid4/5/6 has limitation. If discard request size is small, we do
discard for one disk, but we need calculate parity and write parity disk. To
correctly calculate parity, zero_after_discard must be guaranteed.
I'm wondering if we could use the new bad blocks facility to mark
discarded ranges so we don't necessarily need determinate data after
discard.
...but I have not looked into it beyond that.
--
Dan
No.
The bad blocks framework can only store a limited number of bad ranges - 512
in the current implementation.
That would not be an acceptable restriction for discarded ranges.
You would need a bitmap of some sort if you wanted to record discarded
regions.
http://neil.brown.name/blog/20110216044002#5
This appears to remove the unnecessary resync for discarded range after
a crash
or discard error, eg an enhancement. From my understanding, it can't
remove the
limitation I mentioned in the patch. For raid5, we still need discard a
whole
stripe (discarding one disk but writing parity disk isn't good).
Thanks,
Shaohua
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