Re: write-behind has no measurable effect?

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

a question here...
what happen if all disks are write-mostly and just ssd is write-behind?

why?
write-behind is an async feature (md only return ok to filesystem if
non write-behind disks are sync writen)
write-mostly is a read_balance optimization (ony read from that device
if all non-write-mostly devices fail)

making all disks write-mostly could allow us to use write-behind on
slowest(s) disk(s)

another idea...
could we change raid1 write code? how?
if a total of X write are done, return ok to filesystem, all other
devices are marked as write-behind (automatic write-behind) after sync
writes disks are marked as non-write-behind again
maybe a optimization is: what disk MUST be sync(only non
write-behind), what disk MUST BE async (only write-behind), what disk
can be async/sync (any write-behind type)


another question...
can read balance use write-mostly device in a very busy system without
failed devices (all mirrors are in sync)?


2011/2/16 Andras Korn <korn@raidlist.elan.rulez.org>:
> On Tue, Feb 15, 2011 at 06:10:17AM -0300, Roberto Spadim wrote:
>
>> andras could you make some benchmarks to raid1 with round robin read balance?
>> at this site:
>> www.spadim.com.br/raid1/
>
> For the record: we did some benchmarks and while the patch shows promise and
> seems to cause no problems, it resulted in no measurable performance
> increase for a RAID1 array composed of an SSD and two 7200rpm HDDs.
>
> Andras
>
> --
>                     Andras Korn <korn at elan.rulez.org>
>                 If it ain't broken, play with it until it is.
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-- 
Roberto Spadim
Spadim Technology / SPAEmpresarial
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