Re: Unbelievably _good_ write performance: RHEL5.4 mirror

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

Michael Evans wrote:
>> So, you're saying that the MD layer is doing it's own buffering?  Are
>> you sure?  With the system cache disabled and the drive (and block
>> device driver) cache disabled, there should be no reason to require
>> synchronous I/O, unless, as you suggest, the MD layer is broken.
>>
>> You're saying that both O_DIRECT and O_SYNC must be used to disable
>> cache effects. Why then are there two separate flags and not just one?
>>  Synchronous is a very different behavior that is not necessary for
>> this test and put additional requirements that are not needed for this
>> test.
>>
>> Chris
>>
> 
> Reading the manual page it seems O_DIRECT explicitly minimizes any
> attempts at extra copying; not explicitly disabling buffers, merely
> not adding more.  In another mail thread this tweak-able was
> discussed:
> 
> echo 0 > /sys/block/md*/md/stripe_cache_size

stripe_cache_size only exists for raid4, raid5 and raid6.
It never existed for other raid levels because it's pointless.

/mjt