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@xxxxxxxxxx IBM Deutschland Entwicklung GmbH Vorsitzender des Aufsichtsrats: Martin Jetter Geschäftsführung: Herbert Kircher Sitz der Gesellschaft: Böblingen Registergericht: Amtsgericht Stuttgart, HRB 243294 ---------------------------------------------------------------------------------- When all other means of communication fail, try words. dm-devel-bounces@xxxxxxxxxx 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 > > -- > dm-devel mailing list > dm-devel@xxxxxxxxxx > https://www.redhat.com/mailman/listinfo/dm-devel -- dm-devel mailing list dm-devel@xxxxxxxxxx https://www.redhat.com/mailman/listinfo/dm-devel
