Neil Brown wrote:
On Tuesday April 18, mjt@xxxxxxxxxx wrote:
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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
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