On Mon, Jul 16, 2007 at 12:41:15PM +1000, David Chinner wrote: > On Fri, Jul 13, 2007 at 03:36:46PM -0400, Justin Piszcz wrote: > > On Fri, 13 Jul 2007, Jon Collette wrote: > > > > >Wouldn't Raid 6 be slower than Raid 5 because of the extra fault tolerance? > > > http://www.enterprisenetworksandservers.com/monthly/art.php?1754 - 20% > > >drop according to this article > > > > > >His 500GB WD drives are 7200RPM compared to the Raptors 10K. So his > > >numbers will be slower. > > >Justin what file system do you have running on the Raptors? I think thats > > >an interesting point made by Joshua. > > > > I use XFS: > > When it comes to bandwidth, there is good reason for that. > > > >>>Trying to stick with a supported config as much as possible, I need to > > >>>run ext3. As per usual, though, initial ext3 numbers are less than > > >>>impressive. Using bonnie++ to get a baseline, I get (after doing > > >>>'blockdev --setra 65536' on the device): > > >>>Write: 136MB/s > > >>>Read: 384MB/s > > >>> > > >>>Proving it's not the hardware, with XFS the numbers look like: > > >>>Write: 333MB/s > > >>>Read: 465MB/s > > >>> > > Those are pretty typical numbers. In my experience, ext3 is limited to about > 250MB/s buffered write speed. It's not disk limited, it's design limited. e.g. > on a disk subsystem where XFS was getting 4-5GB/s buffered write, ext3 was doing > 250MB/s. > > http://oss.sgi.com/projects/xfs/papers/ols2006/ols-2006-paper.pdf > > If you've got any sort of serious disk array, ext3 is not the filesystem > to use.... To show what the difference is, I used blktrace and Chris Mason's seekwatcher script on a simple, single threaded dd command on a 12 disk dm RAID0 stripe: # dd if=/dev/zero of=/mnt/scratch/fred bs=1024k count=10k; sync http://oss.sgi.com/~dgc/writes/ext3_write.png http://oss.sgi.com/~dgc/writes/xfs_write.png You can see from the ext3 graph that it comes to a screeching halt every 5s (probably when pdflush runs) and at all other times the seek rate is >10,000 seeks/s. That's pretty bad for a brand new, empty filesystem and the only way it is sustained is the fact that the disks have their write caches turned on. ext4 will probably show better results, but I haven't got any of the tools installed to be able to test it.... The XFS pattern shows consistently an order of magnitude less seeks and consistent throughput above 600MB/s. To put the number of seeks in context, XFS is doing 512k I/Os at about 1200-1300 per second. The number of seeks? A bit above 10^3 per second or roughly 1 seek per I/O which is pretty much optimal. Cheers, Dave. -- Dave Chinner Principal Engineer SGI Australian Software Group - To unsubscribe from this list: send the line "unsubscribe linux-raid" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
