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. -Justin -- 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
