On Thu, May 16, 2013 at 02:51:46PM -0700, Patrick Ng wrote: > Hi all, > > I have installed Linux kernel 3.9.2 in my Fedora 18. When I ran FIO in bcache write-through mode, the performance was good, but running a similar test in bcache write-back mode, I got very poor performance. > Did I miss any options when setting the write-back mode? > > Environment > =========== > 500G backing store at /dev/sdb1 > 150G SSD at /dev/sdc1 > > Write-through mode > ================== > make-bcache -B /dev/sdb1 -C /dev/sdc1 > echo /dev/sdb1 > /sys/fs/bcache/register > echo /dev/sdc1 > /sys/fs/bcache/register > mkfs.ext4 /dev/bcache0 > mount /dev/bcache0 /mnt/bcache > > > Running FIO with random, 100% read, iosize=4096, queue depth=16 > the result is approx. 65k IOPS > > > Write-backe mode (setup is almost the same as write-through except the extra step at the end "echo writeback ...") > ================ > make-bcache -B /dev/sdb1 -C /dev/sdc1 > echo /dev/sdb1 > /sys/fs/bcache/register > echo /dev/sdc1 > /sys/fs/bcache/register > mkfs.ext4 /dev/bcache0 > mount /dev/bcache0 /mnt/bcache > echo writeback > /sys/block/bcache0/bcache/cache_mode > > Running FIO with random, 100% read, iosize=4096, queue depth=16 > the result is approx. 12k IOPS > > Running FIO with random, 80% read 20% write, iosize=4096, queue depth=16 > the result is approx. 2k IOPS You're probably getting bit by the congested bypass - try setting congested_read_threshold_us and congested_write_threshold_us both to 0. I wish there was a better solution to the congestion bypass stuff, it's a hard problem... -- To unsubscribe from this list: send the line "unsubscribe linux-bcache" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
