Roch - PAE wrote: > Neil Perrin writes: > > > > > > Joe Little wrote: > > > On Nov 16, 2007 9:13 PM, Neil Perrin <[EMAIL PROTECTED]> wrote: > > >> Joe, > > >> > > >> I don't think adding a slog helped in this case. In fact I > > >> believe it made performance worse. Previously the ZIL would be > > >> spread out over all devices but now all synchronous traffic > > >> is directed at one device (and everything is synchronous in NFS). > > >> Mind you 15MB/s seems a bit on the slow side - especially is > > >> cache flushing is disabled. > > >> > > >> It would be interesting to see what all the threads are waiting > > >> on. I think the problem maybe that everything is backed > > >> up waiting to start a transaction because the txg train is > > >> slow due to NFS requiring the ZIL to push everything synchronously. > > >> > > > > > > I agree completely. The log (even though slow) was an attempt to > > > isolate writes away from the pool. I guess the question is how to > > > provide for async access for NFS. We may have 16, 32 or whatever > > > threads, but if a single writer keeps the ZIL pegged and prohibiting > > > reads, its all for nought. Is there anyway to tune/configure the > > > ZFS/NFS combination to balance reads/writes to not starve one for the > > > other. Its either feast or famine or so tests have shown. > > > > No there's no way currently to give reads preference over writes. > > All transactions get equal priority to enter a transaction group. > > Three txgs can be outstanding as we use a 3 phase commit model: > > open; quiescing; and syncing. > > That makes me wonder if this is not just the lack of write > throttling issue. If one txg is syncing and the other is > quiesced out, I think it means we have let in too many > writes. We do need a better balance. > > Neil is it correct that reads never hit txg_wait_open(), but > they just need an I/O scheduler slot ?
Yes, they don't modify any meta data (except access time which is handled separately). I'm less clear about what happens further down in the DMU and SPA. _______________________________________________ zfs-discuss mailing list zfs-discuss@opensolaris.org http://mail.opensolaris.org/mailman/listinfo/zfs-discuss
