Re: RAID-5 streaming read performance

[Dan Christensen <jdc@uwo.ca>]

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

> On Thu, 2005-07-14 at 19:29 -0400, Mark Hahn wrote:
>>
>> > i also want a way to clear part of the whole page cache by file id. :)
>> 
>> understandably, kernel developers are don't high-prioritize this sort of 
>> not-useful-for-normal-work feature.
> agree.

Clearing just part of the page cache sounds too complicated to be
worth it, but clearing it all seems reasonable;  some kernel developers
spend time doing benchmarks too!

>> > Dan Christensen wrote:
>> >
>> > > I'm really surprised there isn't something in /proc you can use to
>> > > clear or disable the cache.  Would be very useful for benchmarking!
>> 
>> I assume you noticed "blockdev --flushbufs", no?  it works for me 

I had tried this and noticed that it didn't work for files on a
filesystem.  But it does seem to work for block devices.  That's
great, thanks.  I didn't realize the cache was so complicated;
it can be retained for files but not for the block device underlying
those files!  

> a test i did show that even you have sda and sdb to form a raid0,
> the page cache for sda and sdb will not be used by raid0. kind of
> funny.

I thought I had noticed raid devices making use of cache from
underlying devices, but a test I just did agrees with your result, for
both RAID-1 and RAID-5.  Again, this seems odd.  Shouldn't the raid
layer take advantage of a block that's already in RAM?  I guess this
won't matter in practice, since you usually don't read from both a
raid device and an underlying device.

Dan