On 8/21/2012 9:51 AM, Miquel van Smoorenburg wrote:

On 08/20/2012 01:34 AM, Stan Hoeppner wrote:

quoted

I'm glad you jumped in David.  You made a critical statement of fact
below which clears some things up.  If you had stated it early on,
before Miquel stole the thread and moved it to LKML proper, it would
have short circuited a lot of this discussion.  Which is:

I'm sorry about that, that's because of the software that I use to
follow most mailinglist. I didn't notice that the discussion was cc'ed
to both lkml and l-r. I should fix that.

Oh, my bad.  I thought it was intentional.

Don't feel too bad about it.  When I tried to copy lkml back in on the
one message I screwed up as well.  I though Tbird had filled in the full
address but it didn't.

quoted

Thus my original statement was correct, or at least half correct[1], as
it pertained to md/RAID6.  Then Miquel switched the discussion to
md/RAID5 and stated I was all wet.  I wasn't, and neither was Dave
Chinner.  I was simply unaware of this md/RAID5 single block write RMW
shortcut

Well, all I tried to say is that a small write of, say, 4K, to a
raid5/raid6 array does not need to re-write the whole stripe (i.e.
chunksize * nr_disks) but just 4K * nr_disks, or the RMW variant of that.

And I'm glad you did.  Before that I didn't know about these efficiency
shortcuts and exactly how md does writeback on partial stripe updates.

Even with these optimizations, a default 512KB chunk is too big, for the
reasons I stated, the big one being the fact that you'll rarely fill a
full stripe, meaning nearly every write will incur an RMW cycle.

-- 
Stan