On Mon, Feb 7, 2011 at 1:51 PM, Brandon High <bh...@freaks.com> wrote: > On Mon, Feb 7, 2011 at 10:29 AM, Yi Zhang <yizhan...@gmail.com> wrote: >> I already set primarycache to metadata, and I'm not concerned about >> caching reads, but caching writes. It appears writes are indeed cached >> judging from the time of 2.a) compared to UFS+directio. More >> specifically, 80MB/2s=40MB/s (UFS+directio) looks realistic while >> 80MB/0.11s=800MB/s (ZFS+primarycache=metadata) doesn't. > > You're trying to force a solution that isn't relevant for the > situation. ZFS is not UFS, and solutions that are required for UFS to > work correctly are not needed with ZFS. > > Yes, writes are cached, but all the POSIX requirements for synchronous > IO are met by the ZIL. As long as your storage devices, be they SAN, > DAS or somewhere in between respect cache flushes, you're fine. If you > need more performance, use a slog device that respects cache flushes. > You don't need to worry about whether writes are being cached, because > any data that is written synchronously will be committed to stable > storage before the write returns. > > -B > > -- > Brandon High : bh...@freaks.com > Maybe I didn't make my intention clear. UFS with directio is reasonably close to a raw disk from my application's perspective: when the app writes to a file location, no buffering happens. My goal is to find a way to duplicate this on ZFS.
Setting primarycache didn't eliminate the buffering, using O_DSYNC (whose side effects include elimination of buffering) made it ridiculously slow: none of the things I tried eliminated buffering, and just buffering, on ZFS. >From the discussion so far my feeling is that ZFS is too different from UFS that there's simply no way to achieve this goal... _______________________________________________ zfs-discuss mailing list zfs-discuss@opensolaris.org http://mail.opensolaris.org/mailman/listinfo/zfs-discuss
