On 01/27/2012 02:06 AM, Richard Sharpe wrote: > Hi, > > Perhaps I am doing something stupid, but I would like to understand > why there is a difference in the following situation. > > I have defined a stripe device thusly: > > "echo 0 17560535040 striped 9 8 /dev/sdd 0 /dev/sde 0 /dev/sdf 0 > /dev/sdg 0 /dev/sdh 0 /dev/sdi 0 /dev/sdj 0 /dev/sdk 0 /dev/sdl 0 | > dmsetup create stripe_dev" > > Then is did the following: > > dd if=/dev/zero of=/dev/mapper/stripe_dev bs=262144 count=1000000 > > and I got 880 MB/s > > However, when I changed that command to: > > dd if=/dev/zero of=/dev/mapper/stripe_dev bs=262144 count=1000000 > oflag=direct > > I get 210 MB/s reliably. > > The system in question is a 16 core (probably two CPUs) Intel Xeon > E5620 @2.40Ghz with 64GB of memory and 12 7200PRM SATA drives > connected to an LSI SAS controller but set up as a JBOD of 12 drives. > > Why do I see such a big performance difference? Does writing to the > device also use the page cache if I don't specify DIRECT IO? > Yes. All I/O using read/write calls is going via the pagecache. The only way to circumvent this is to use DIRECT_IO. Cheers, Hannes -- Dr. Hannes Reinecke zSeries & Storage hare@xxxxxxx +49 911 74053 688 SUSE LINUX Products GmbH, Maxfeldstr. 5, 90409 Nürnberg GF: J. Hawn, J. Guild, F. Imendörffer, HRB 16746 (AG Nürnberg) -- dm-devel mailing list dm-devel@xxxxxxxxxx https://www.redhat.com/mailman/listinfo/dm-devel
