Re: linear writes to raid5

[Michael Tokarev <mjt@tls.msk.ru>]

Neil Brown wrote:
> On Tuesday April 18, mjt@tls.msk.ru wrote:
[]
>> I mean, mergeing bios into larger requests makes alot of sense between
>> a filesystem and md levels, but it makes alot less sense to do that
>> between md and physical (fsvo "physical" anyway) disks.
> 
> This seems completely backwards to me, so we must be thinking of
> different things.
> 
> Creating large requests above the md level doesn't make a lot of sense
> to me because there is a reasonable chance that md will just need to
> break the requests up again to submit to different devices lower down.
> 
> Building large requests for the physical disk makes lots of sense
> because you get much better throughput on an e.g. SCSI buss by having
> few large requests rather than many small requests.  But this building
> should be done close to the device so that as much information as
> possible is available about particular device characteristics.
> 
> What is the rationale for your position?

My rationale was that if md layer receives *write* requests not smaller
than a full stripe size, it is able to omit reading data to update, and
can just calculate new parity from the new data.  Hence, combining a
dozen small write requests coming from a filesystem to form a single
request >= full stripe size should dramatically increase performance.

Eg, when I use dd with O_DIRECT mode (oflag=direct) and experiment with
different block size, write performance increases alot when bs becomes
full stripe size.  Ofcourse it decreases again when bs is increased a
bit further (as md starts reading again, to construct parity blocks).

For read requests, it makes much less difference where to combine them.

/mjt