Neil Brown <neilb@xxxxxxx> writes: > For the O_SYNC: > I think this is a RAID1 - is that correct? Hi Neil. It's a RAID10n2 of six disks, but I've also seen the behaviour on a RAID1 of two disks around the time of 2.6.27. > With RAID1, as soon as any IO request arrives, resync is suspended and > as soon as all resync requests complete, the IO is permitted to > proceed. > So normal IO takes absolute precedence over resync IO. > > So I am very surprised to here that O_SYNC writes deadlock > completed. > As O_SYNC writes are serialised, there will be a moment between > every pair when there is no IO pending. This will allow resync to > get one "window" of resync IO started between each pair of writes. > So I can well believe that a sequence of O_SYNC writes are a couple > of orders of magnitude slower when resync is happening than without. > But it shouldn't deadlock completely. > Once you get about 64 sectors of O_SYNC IO through, the resync > should notice and back-off and resync IO will be limited to the > 'minimum' speed. The symptoms seem to be that I can't read or write to /dev/mdX but I can read from the underlying /dev/sd* devices fine, at pretty much full speed. I didn't try writing to them as there's lots of live customer data on the RAID arrays! The configuration is lvm2 (i.e. device-mapper linear targets) on top of md on top of sd, and we've seen the symptoms with the virtual machines accessing the logical volumes configured to open in O_SYNC mode, and with them configured to open in O_DIRECT mode. During the deadlock, cat /proc/mdstat does return promptly (i.e. not blocked), and shows a slow and gradually falling sync rate---I think that there's no sync writing going on either and the drives are genuinely idle. We have to reset the machine to bring it back to life and a graceful reboot fails. Anyway, I see this relatively infrequently, so what I'll try to do is to create a reproducible test case and then follow up to you and the RAID list with that. At the moment, I understand that my reports is a bit anecdotal, and without a proper idea of what conditions are needed to make it happen its pretty much impossible to diagnose or work on! Cheers, Chris. -- To unsubscribe from this list: send the line "unsubscribe linux-ide" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
