Limited write bandwidth from ext3

Sinha_Himanshu at Sinha_Himanshu at
Mon Jun 19 18:18:12 UTC 2006

We are running a benchmark that does single threaded 512 KB writes to  a
LUN on a CLARiiON storage array. The dual Xeon host (Dell 2650) with 4
GB of memory runs RHEL 4U3

We measured the write bandwidth for writes to the block device
corresponding to the lun (e.g. /dev/sdb), a file in an ext2 filesystem
and to a file in an ext3 file system.
		Write b/w for 512 KB writes
Block device	312 MBps
Ext2 file		247 MBps
Ext3 file		130 MBps

We are looking for ways to improve the ext3 file write bandwidth.

Tracing of I/Os at the storage array shows that in the case of ext3
experiment, the workload does not keep the lun busy enough. Every 5
seconds there is an increase in I/O activity that lasts for 2-3 seconds.
The lun then has very low activity for 2-3 seconds. It appears that the
buffers at the host are flushed every 5 seconds and the flushing takes
2-3 seconds. To maximize write bandwidth, we would like to be in a
situation where the buffers are flushed continuously to keep the lun
constanly busy.
That is what we see in the case of the ext2 file. 

In the case of ext2 we also see the host do quite a few ver large writes
(up to 7784 KB).

Himanshu Sinha

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