On Jun 26, 2008, at 3:19 AM, Krishna Kumar2 wrote:
Benny Halevy <bhalevy@xxxxxxxxxxx> wrote on 06/23/2008 06:10:40 PM:
Apparently the file is cached. You needed to restart nfs
and remount the file system to make sure it isn't before reading it.
Or, you can create a file larger than your host's cache size so
when you write (or read) it sequentially, its tail evicts its head
out of the cache. This is a less reliable method, yet creating a
file about 25% larger than the host's memory size should work for
you.
I did a umount of all filesystems and restart NFS before testing. Here
is the result:
Local:
Read: 69.5 MB/s
Write: 70.0 MB/s
NFS of same FS mounted loopback on same system:
Read: 29.5 MB/s (57% drop)
Write: 27.5 MB/s (60% drop)
The drops seems exceedingly high. How can I figure out the source of
the
problem? Even if it is as general as to be able to state: "Problem
is in
the NFS client code" or "Problem is in the NFS server code", or
"Problem
can be mitigated by tuning" :-)
It's hard to say what might be the problem just by looking at
performance results.
You can look at client-side NFS and RPC performance metrics using some
prototype Python tools that were just added to nfs-utils. The scripts
themselves can be downloaded from:
http://oss.oracle.com/~cel/Linux-2.6/2.6.25
but unfortunately they are not fully documented yet so you will have
to approach them with an open mind and a sense of experimentation.
You can also capture network traces on your loopback interface to see
if there is, for example, unexpected congestion or latency, or if
there are other problems.
But for loopback, the problem is often that the client and server are
sharing the same physical memory for caching data. Analyzing your
test system's physical memory utilization might be revealing.
Otherwise, you should always expect some performance degradation when
comparing NFS and local disk. 50% is not completely unheard of. It's
the price paid for being able to share your file data concurrently
among multiple clients.
--
Chuck Lever
chuck[dot]lever[at]oracle[dot]com
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