On Wed, Mar 27, 2013 at 11:03:52AM +0100, Javier Marcet wrote: > Hi, > > I've been using bcache since last summer, caching a 4x3TB raid5. > > Last week one of my disks died so after replacing it, I also got a new > SSD and began to do some tests. There is something which, while not > proving anything, worries me anyway. Better than describing it, just > see this (caches were flushed before each hdparm): I think I may have finally figured this one out - bcache wasn't setting readahead_pages, and raid5/6 needs it to be set based on the number of devices in the array. Can you try the version I just pushed? > > # hdparm -tT /dev/md0 > > /dev/md0: > Timing cached reads: 23860 MB in 2.00 seconds = 11941.98 MB/sec > Timing buffered disk reads: 996 MB in 3.00 seconds = 331.85 MB/sec > > # hdparm -tT /dev/bcache0 > > /dev/bcache0: > Timing cached reads: 22944 MB in 2.00 seconds = 11482.63 MB/sec > Timing buffered disk reads: 488 MB in 3.00 seconds = 162.49 MB/sec > > This is without any caching devices attached, that is, in pass-through > mode. My md raid5 is bottlenecked by the 3Gbps of the SATA controller > but with bcache, even in pass-through mode, seems to have a degrading > effect. > > With a caching device attached, I know random I/O improves, but it > seems I'm degrading sequential access considerably. See now with a > fresh 128GB Crucial M4 SSD attached: > > # hdparm -tT /dev/sdb > > /dev/sdb: > Timing cached reads: 23964 MB in 2.00 seconds = 11994.69 MB/sec > Timing buffered disk reads: 1594 MB in 3.00 seconds = 530.72 MB/sec > > # hdparm -tT /dev/bcache0 > > /dev/bcache0: > Timing cached reads: 23830 MB in 2.00 seconds = 11927.29 MB/sec > Timing buffered disk reads: 626 MB in 3.00 seconds = 208.64 MB/sec > > # hdparm -tT /dev/bcache0 > > /dev/bcache0: > Timing cached reads: 24156 MB in 2.00 seconds = 12089.18 MB/sec > Timing buffered disk reads: 542 MB in 3.05 seconds = 177.53 MB/sec > > I would like to try more suited testing tools directly over the RAID, > without any bcache but it looks like I'd need to recreate the whole > array, sigh. > > I would appreciate any more info about these numbers. > > > -- > Javier Marcet <jmarcet@xxxxxxxxx> > -- > 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 -- 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
