[Lustre-discuss] Reads Starved

[Roger Spellman]

> -----Original Message-----
> From: Peter Grandi [mailto:pg_mh at mh.to.sabi.co.UK]
> Sent: Thursday, June 02, 2011 4:10 PM
> To: Roger Spellman
> Cc: lustre-discuss at lists.lustre.org
> Subject: Re: [Lustre-discuss] Reads Starved
> 
> Unfortunately these numbers are meaningless without an idea of
> the storage system and the access patterns.

The storage system is a Dell MD3200.

> 
> > But, when I have 10 clients doing reads, and a different 10
> > clients doing writes, the write performance barely drops, but
> > the read performance drops to about 150 MB/s.
> 
> This may be entirely the right thing for it to happen.

Each test is doing large block, streaming I/O.  The reads and write are
accessing different files.  I am running 1 thread per client.  If I
increase the thread count on the reads, the performance does increase,
but not nearly to the level of the writes.

I had increased max_rpcs quite large, as this gives excellent
performance.  However, this turns out to be the cause of this read/write
disparity.  The system can queue up many writes, but reads are issued
one at a time.  By decreasing max_rpcs, the read performance increased
considerably.

Roger Spellman
Staff Engineer
Terascala, Inc.
508-588-1501
www.terascala.com <http://www.terascala.com/>