[Lustre-discuss] OSS Cache Size for read optimization

[Andreas Dilger]

On Apr 02, 2009  15:17 -0700, Jordan Mendler wrote:
> I deployed Lustre on some legacy hardware and as a result my (4) OSS's each
> have 32GB of RAM. Our workflow is such that we are frequently rereading the
> same 15GB indexes over and over again from Lustre (they are striped across
> all OSS's) by all nodes on our cluster. As such, is there any way to
> increase the amount of memory that either Lustre or the Linux kernel uses to
> cache files read from disk by the OSS's? This would allow much of the
> indexes to be served from memory on the OSS's rather than disk.

With Lustre 1.8.0 (in late release testing, you could grab
v1_8_0_RC5 from CVS for testing[*]) there is OSS server-side caching
of read and just-written data.  There is a tunable that allows
limiting the maximum file size that is cached on the OSS so that
small files can be cached, and large files will not wipe out the
read cache, /proc/fs/lustre/obdfilter/*/readcache_max_filesize.

Set readcache_max_filesize just large enough to hold your index files
(which are hopefully not too large individually) to maximize your
cache retention.  While the cache eviction is LRU, it may be that
at high IO rates your working set would still be evicted from RAM
if too many other files fall within the cache file size limit.

[*] Note that v1_8_0_RC5 is missing the fix for bug 18659 so is not at
all safe to use on the MDS, v1_8_0_RC6 will have that fix, as does b1_8.

> I see a *lustre.memused_max = 48140176* parameter, but not sure what that
> does. If it matters, my setup is such that each of the 4 OSS's serves 1 OST
> that consists of a software RAID10 across 4 SATA disks internal to that OSS.

That is just reporting the total amount of RAM ever used by the
Lustre code itself (48MB in this case), and has nothing to do with
the cached data.

Cheers, Andreas
--
Andreas Dilger
Sr. Staff Engineer, Lustre Group
Sun Microsystems of Canada, Inc.