[Lustre-discuss] Interpreting iozone measurements

[Michael Kluge]

Hi all,

we have a testbed running with Lustre 1.8.3 and a RTT of ~4ms (10GE
network cards everywhere) for a ping between client and servers. If I
have read the iozone source code correctly, iozone reports bandwidth in
KB/s and includes the time for the open() call, but not for close(). If
I write a single 64 byte file (with one I/O request), iozone tells me
something like '295548', which means ~295 MB/s. Dividing the file size
by the bandwidth, I get the time that was needed to create the file and
write the 64 bytes (with a single request). In this case, the time is
about 0,2 micro seconds which is way below the RTT. 

That mean for a Lustre file system, if I create a file and write 64
bytes, the client sends two(?) RPCs to the server and does not wait for
the completion. Is this correct? But it will wait for the completion of
both RPCs when the file ist closed?

The numbers look different when I disable the client side cache by
setting max_dirty_mb to 0.


Regards, Michael

-- 

Michael Kluge, M.Sc.

Technische Universität Dresden
Center for Information Services and
High Performance Computing (ZIH)
D-01062 Dresden
Germany

Contact:
Willersbau, Room A 208
Phone:  (+49) 351 463-34217
Fax:    (+49) 351 463-37773
e-mail: michael.kluge at tu-dresden.de
WWW:    http://www.tu-dresden.de/zih
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