Indeed the scrub seems to take too much resources from a live system. For instance i have a server with 24 disks (SATA 1TB) serving as NFS store to a linux machine holding user mailboxes. I have around 200 users, with maybe 30-40% of active users at the same time. As soon as the scrub process kicks in, linux box starts to give messages like " nfs server not available" and the users start to complain that the Outlook gives "connection timeout". Again, as soon as the scrub process stops everything comes to normal. So for me, it's real issue the fact that the scrub takes so many resources of the system, making it pretty much unusable. In my case i did a *workaround, *where basically i have zfs send/receive from this server to another server and the scrub process is now running on the second server. I don't know if this such a good idea, given the fact that i don't know for sure if the scrub process in the secondary machine will be usefull in case of data corruption...but so far so good , and it's probably better than nothing. I still remember before ZFS , that any good RAID controller would have a background consistency check task, and such a task would be possible to assign priority , like "low, medium, high" ...going back to ZFS what's the possibility of getting this feature as well?
Just out as curiosity , the Sun OpenStorage appliances , or Nexenta based ones, have any scrub task enabled by default ? I would like to get some feedback from users that run ZFS appliances regarding the impact of running a scrub on their appliances. Bruno On 28-4-2010 22:39, David Dyer-Bennet wrote: > 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%! > > -- This message has been scanned for viruses and dangerous content by MailScanner, and is believed to be clean.
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