Re: RAID-5 streaming read performance

[Dan Christensen <jdc@uwo.ca>]

Ming Zhang <mingz@ele.uri.edu> writes:

> On Wed, 2005-07-13 at 23:58 -0400, Dan Christensen wrote:
>
>> But I still don't understand why the md layer can't do better.  If I
>> turn off readahead of the raw devices, and keep it for the raid
>> device, then parity blocks should never be requested, so they
>> shouldn't use any bus/controller bandwidth.  And even if each drive is
>> only acting at 75% efficiency, the four drives should still be able to
>> saturate the bus/controller.  So I can't figure out what's going on
>> here.
> 
> when read, i do not think MD will read parity at all. but since parity
> is on all disk, there might be a seek here.

Yes, there will be a seek, or internal drive readahead, so each drive
will operate at around 75% efficiency.  But since that shouldn't
affect bus/controller traffic, I still would expect to get over
100MB/s with my hardware.

>> Also, is there a way to disable caching of reads?
> 
> after you run your code, check the meminfo, the cached value might be
> much lower than u expected. my feeling is that linux page cache will
> discard all cache if last file handle closed.

Ming Zhang <mingz@ele.uri.edu> writes:

> my problem here. this only apply to sdX not mdX. pls ignore this.

I'm not sure what you mean.  For reads from sdX, mdX, files on sdX
or files on mdX, the cache is retained.  So it's necessary to clear
this cache to get valid timing results.

Dan