Limited write bandwidth from ext3
Herta Van den Eynde
herta.vandeneynde at cc.kuleuven.be
Wed Jun 21 14:34:55 UTC 2006
Sinha_Himanshu at emc.com wrote:
> 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
> Ext3-users mailing list
> Ext3-users at redhat.com
Sounds like you are describing the default ext3 sync'ing feature. ext3
has a mount option "commit", with default value 5 seconds. It causes
all data and metadata to be sync'ed every 5. (Cf. 'man mount')
I'm not an expert, so I'm not sure what the potential impact might be
(other than delaying sync times) if you change this value.
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