On Wed, April 28, 2010 10:16, Eric D. Mudama wrote: > 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.
The situations being mentioned are much worse than what seem reasonable tradeoffs to me. Maybe that's because my intuition is misleading me about what's available. But if the normal workload of a system uses 25% of its sustained IOPS, and a scrub is run at "low priority", I'd like to think that during a scrub I'd see a little degradation in performance, and that the scrub would take 25% or so longer than it would on an idle system. There's presumably some inefficiency, so the two loads don't just add perfectly; so maybe another 5% lost to that? That's the big uncertainty. I have a hard time believing in 20% lost to that. Do you think that's a reasonable outcome to hope for? Do you think ZFS is close to meeting it? People with systems that live at 75% all day are obviously going to have more problems than people who live at 25%! -- David Dyer-Bennet, d...@dd-b.net; http://dd-b.net/ Snapshots: http://dd-b.net/dd-b/SnapshotAlbum/data/ Photos: http://dd-b.net/photography/gallery/ Dragaera: http://dragaera.info _______________________________________________ zfs-discuss mailing list zfs-discuss@opensolaris.org http://mail.opensolaris.org/mailman/listinfo/zfs-discuss
