On Mon, 30 Aug 2010, Justin Maggard wrote: > On Mon, Aug 30, 2010 at 5:37 PM, Ted Ts'o <tytso@xxxxxxx> wrote: > > On Mon, Aug 30, 2010 at 04:49:58PM -0400, Bill Fink wrote: > >> > Thanks for reporting it. I'm going to have to take a closer look at > >> > why this makes a difference. I'm going to guess though that what's > >> > going on is that we're posting writes in such a way that they're no > >> > longer aligned or ending at the end of a RAID5 stripe, causing a > >> > read-modify-write pass. That would easily explain the write > >> > performance regression. > >> > >> I'm not sure I understand. How could calling or not calling > >> ext4_num_dirty_pages() (unpatched versus patched 2.6.35 kernel) > >> affect the write alignment? > > > > Suppose you have 8 disks, with stripe size of 16k. Assuming that > > you're only using one parity disk (i.e., RAID 5) and no spare disks, > > that means the optimal I/O size is 7*16k == 112k. If we do a write > > which is smaller than 112k, or which is not a multiple of 112k, then > > the RAID subsystem will need to do a read-modify-write to update the > > parity disk. Furthermore, the write had better be aligned on an 112k > > byte boundary. The block allocator will guarantee that block #0 is > > aligned on a 112k block, but writes have to also be right size in > > order to avoid the read-modify-write. > > > > If we end up doing very small writes, then it can end up being quite > > disatrous for write performance. > > I'd have to agree that this is likely the case. Just to add a little > more data here, I tried the same 32GB dd test against a 12-disk MD > RAID 6 64k chunk array today with and without the patch (although > against a 2.6.33.7 kernel), and my write performance dropped from > ~420MB/sec down to 350MB/sec when I used the patched kernel. I'm curious. Since you're using 12 disks where I was only using 8, I'm wondering what performance you would get if you changed the multiplier to say 16, i.e. desired_nr_to_write = wbc->nr_to_write * 16; It seems you should be getting better than 420 MB/sec on a 12-disk raid, although perhaps the overhead of doing RAID6 is an issue. I use md RAID0 to combine 2 of the hardware RAID5 arrays (total of 16 disks), and I'm seeing (with my patch) 1.3 GB/sec write performance. -Bill -- To unsubscribe from this list: send the line "unsubscribe linux-ext4" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
