On 28 April, 2010 - Eric D. Mudama sent me these 1,6K bytes: > On Wed, Apr 28 at 1:34, Tonmaus wrote: >>> Zfs scrub needs to access all written data on all >>> disks and is usually >>> disk-seek or disk I/O bound so it is difficult to >>> keep it from hogging >>> the disk resources. A pool based on mirror devices >>> will behave much >>> more nicely while being scrubbed than one based on >>> RAIDz2. >> >> Experience seconded entirely. I'd like to repeat that I think we >> need more efficient load balancing functions in order to keep >> housekeeping payload manageable. Detrimental side effects of scrub >> should not be a decision point for choosing certain hardware or >> redundancy concepts in my opinion. > > While there may be some possible optimizations, i'm sure everyone > would love the random performance of mirror vdevs, combined with the > redundancy of raidz3 and the space of a raidz1. However, as in all > systems, there are tradeoffs. > > To scrub a long lived, full pool, you must read essentially every > sector on every component device, and if you're going to do it in the > order in which your transactions occurred, it'll wind up devolving to > random IO eventually. > > You can choose to bias your workloads so that foreground IO takes > priority over scrub, but then you've got the cases where people > complain that their scrub takes too long. There may be knobs for > individuals to use, but I don't think overall there's a magic answer.
We have one system with a raidz2 of 8 SATA disks.. If we start a scrub, then you can kiss any NFS performance goodbye.. A single mkdir or creating a file can take 30 seconds.. Single write()s can take 5-30 seconds.. Without the scrub, it's perfectly fine. Local performance during scrub is fine. NFS performance becomes useless. This means we can't do a scrub, because doing so will basically disable the NFS service for a day or three. If the scrub would be less agressive and take a week to perform, it would probably not kill the performance as bad.. /Tomas -- Tomas Ögren, st...@acc.umu.se, http://www.acc.umu.se/~stric/ |- Student at Computing Science, University of Umeå `- Sysadmin at {cs,acc}.umu.se _______________________________________________ zfs-discuss mailing list zfs-discuss@opensolaris.org http://mail.opensolaris.org/mailman/listinfo/zfs-discuss
