Re: Mirrored volume peformance questions

[Roberto Spadim <roberto@spadim.com.br>]

i think that a good add to today read balance could be add disk
acess_time (1/rpm, 0 for 'non rotational', 1 for unknow)
head_distance * acess_time, could allow a better select of disks or
mix of 7200/10000/15000 rpm disks
maybe it can be read from disk information at assemble time, i don't
know if disk speed (rpm) can be read from device block (some disks
display information with hdparm and smartctl)
it's not a 1000% improvement, it's 1% at high read/write load

2011/5/4 Keld Jørn Simonsen <keld@keldix.com>:
> On Wed, May 04, 2011 at 09:42:40AM +0200, David Brown wrote:
>> raid10,far is better for sequential reads - it gives better-than-raid0
>> performance on average since it will do striped reads from the faster
>> outer tracks.  And for multi-threaded reads, it should also be a little
>> faster than other raid10 layouts (and raid1, which is much the same as
>> raid10,near).  Since it prefers to get the data from the outer half, you
>> get the benefits of short-stroking your disks - faster transfer speeds
>> and less head movement.
>>
>> The cost of raid10,far is greater head movement for writes - but that is
>> not the OP's main concern.
>
> yes, in theory this is so. But two reasons almost eliminates this in
> practice. First, the processes do not wait for completion of the IO of
> writes, the processes only deliver the data to the file buffer cache of
> the kernel, which then periodically flushes the data to the disk drives.
> Second, the flushing of the data is ordered so that the collected data
> buffers are written as much sequentially as possible to the drives.
> This goes for all Linux MD RAID1/RAID10 layouts. Given that random
> writes are random over the whole set of drives, for any mirrored
> raid1/raid10 layout, the flushing of the data is about the same.
>
> best regards
> keld
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-- 
Roberto Spadim
Spadim Technology / SPAEmpresarial
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