HP StorageWorks Scalable File Share - Manual (Page 49)

Figure A-3 shows single stream performance for a single process writing and then reading a
single 8GB file. The file was written in a directory with a stripe width of 1MB and stripe count
as shown. The client cache was purged after the write and before the read.
Figure A-3 Single Stream Throughput
For a file written on a single OST (a single RAID volume), throughput is in the neighborhood of
200MB per second. As the stripe count is increased, spreading the load over more OSTs,
throughput increases. Single stream writes top out above 400MB per second and reads exceed
700MB per second.
Figure A-4 compares write performance in three cases. First is a single process writing to N OSTs,
as shown in the previous figure. Second is N processes each writing to a different OST. And
finally, N processes to different OSTs using direct I/O.
Figure A-4 Single Client, Multi-Stream Write Throughput
For stripe counts of four and above, writing with separate processes has a higher total throughput
than a single process. The single process itself can be a bottleneck. For a single process writing
to a single stripe, throughput is lower with direct I/O. This is due to the fact that the direct I/O
write can only send one RPC to the OST at a time, so the I/O pipeline is not kept full.
For stripe counts of 8 and 16, using direct I/O and separate processes yields the highest throughput.
The overhead of managing the client cache lowers throughput, and using direct I/O eliminates
this overhead.
A.2 Single Client Performance 49