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Re: [dm-devel] Size affects performance?



If you use dd for performance tests I would suggest to use oflags=sync. 
Otherwise there might be caching effects.

Mit freundlichem Gruß / Regards,
Stefan Bader

SW Linux on zSeries Development
Stefan Bader de ibm com

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dm-devel-bounces redhat com wrote on 01.08.2007 05:03:52:

> Hi,
> 
> I've noticed that the size of a device seems to affect its performance.
> Is this expected behaviour?
> 
> For example, on a device that is exactly 1GB, I can transfer at 16.4 
> MB/s:
> 
> flanders1:~# dmsetup create test
> 0 2097152 linear /dev/sda6 0
> 
> flanders1:~# dd if=/dev/zero of=/dev/mapper/test bs=1K count=1048576
> 1048576+0 records in
> 1048576+0 records out
> 1073741824 bytes (1.1 GB) copied, 65.4846 seconds, 16.4 MB/s
> 
> 
> Yet, if I resize the device to be one sector larger, the same 
> transfer happens at 59.0 MB/s:
> 
> flanders1:~# dmsetup create test
> 0 2097153 linear /dev/sda6 0
> 
> flanders1:~# dd if=/dev/zero of=/dev/mapper/test bs=1K count=1048576
> 1048576+0 records in
> 1048576+0 records out
> 1073741824 bytes (1.1 GB) copied, 18.1887 seconds, 59.0 MB/s
> 
> 
> For reference, the underlying device seems capable of ~ 60 MB/s:
> 
> flanders1:~# dd if=/dev/zero of=/dev/sda6 bs=1K count=1048576
> 1048576+0 records in
> 1048576+0 records out
> 1073741824 bytes (1.1 GB) copied, 18.1325 seconds, 59.2 MB/s
> 
> 
> I've noticed that the problem either doesn't occur or isn't as 
> pronounced if different block sizes are used (ie. bs=4K).
> I'm running a fairly recent kernel: git tag v2.6.23-rc1-g6a30235
> 
> 
> Thanks for any help,
> 
> 
> Andrew Miklas
> 
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