On Fri, Nov 26, 2010 at 09:16:22AM +0100, Emmanuel Florac wrote: > Le Thu, 25 Nov 2010 16:57:00 -0600 vous écriviez: > > > Looking at the stripe size, which is equal to 64 sectors per array > > member drive (448 sectors total), how exactly is a sub 4KB mail file > > (8 sectors) going to be split up into equal chunks across a 224KB RAID > > stripe? > > It won't, it will simply end on one drive (actually one mirror). > However because the mirrors are striped together, all drives in the > array will be sollicited in my experience, that's why you need at least > as many writing threads as there are stripes to reach the top IOPS. In > your case, writing 56 4K files simultaneously will effectively write on > all drives at once, hopefully (depends upon the filesystem allocation > policy though). > > > Does 220KB of the stripe merely get wasted? > > It's not wasted, it just remains unallocated. What's wasted is > potential IO performance. No, that's wrong. I don't have the time to explain the intricacies of how XFS packs small files together, but it does. You can observe the result by unpacking a kernel tarball and looking at the layout with xfs_bmap if you really want to... FWIW, for workloads that do random, small IO, XFS works best when you _turn off_ aligned allocation and just let it spray the IO at the disks. This works best if you are using RAID 0/1/10. All the numbers I've been posting are with aligned allocation turned off (i.e. no sunit/swidth set). > What appears from the benchmarks I ran along the year is that anyway > you turn it, whatever caching, command tag queuing and reordering > your're using, a single thread can't reach maximal IOPS throughput on > an array, i. e. writing on all drives simultaneously; a single thread > writing to the fastest RAID 10 with 4K or 8K IOs can't do much better > than with a single drive, 200 to 300 IOPS for a 15k drive. Assuming synchronous IO. If you are doing async IO, a single CPU should be able to keep hundreds of SRDs (Spinning Rust Disks) busy... Cheers, Dave. -- Dave Chinner david@xxxxxxxxxxxxx _______________________________________________ xfs mailing list xfs@xxxxxxxxxxx http://oss.sgi.com/mailman/listinfo/xfs
