Re: O_DIRECT to md raid 6 is slow

[Stan Hoeppner <stan@hardwarefreak.com>]

On 8/21/2012 9:51 AM, Miquel van Smoorenburg wrote:
> On 08/20/2012 01:34 AM, Stan Hoeppner wrote:
>> 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.

>> 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