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Chapter 4. Tuning the operating system 119
Draft Document for Review May 4, 2007 11:35 am 4285ch04.fm
Figure 4-9 Impact of nr_requests on the Deadline elevator (random write ReiserFS)
A larger request queue may be offering a higher throughput for workloads that write many
small files. As it can be seen in the graphic displayed in Figure 4-10 a setting of 8192 offers
the highest levels of performance for I/O sizes of up to 16 kB. At 64 kB the analyzed value of
nr_requests from 64 up to 8192 offer about equal performance. However as the I/O size
increases, smaller levels of nr_requests will in most cases result in superior performance. The
number of requests can be changed via the following command:
Example 4-13 Changing nr_requests
# echo 64 > /sys/block/sdb/queue/nr_requests
0
20000
40000
60000
80000
100000
120000
140000
kB/sec
4 8 16 32 64 128 256 512 1024 2048
kB/op
128 nr_requests
64 nr_requests
512 nr_requests
2028 nr_requests