comment below... On May 19, 2010, at 7:50 AM, John Hoogerdijk wrote:
>>> From: zfs-discuss-boun...@opensolaris.org >> [mailto:zfs-discuss- >>> boun...@opensolaris.org] On Behalf Of John >> Hoogerdijk >>> >>> I'm building a campus cluster with identical >> storage in two locations >>> with ZFS mirrors spanning both storage frames. Data >> will be mirrored >>> using zfs. I'm looking for the best way to add log >> devices to this >>> campus cluster. >> >> Either I'm crazy, or I completely miss what you're >> asking. You want to have >> one side of a mirror attached locally, and the other >> side of the mirror >> attached ... via iscsi or something ... across the >> WAN? Even if you have a >> really fast WAN (1Gb or so) your performance is going >> to be terrible, and I >> would be very concerned about reliability. What >> happens if a switch reboots >> or crashes? Then suddenly half of the mirror isn't >> available anymore >> (redundancy is degraded on all pairs) and ... Will it >> be a degraded mirror? >> Or will the system just hang, waiting for iscsi IO to >> timeout? When it >> comes back online, will it intelligently resilver >> only the parts which have >> changed since? Since the mirror is now broken, and >> local operations can >> happen faster than the WAN can carry them across, >> will the resilver ever >> complete, ever? I don't know. >> >> anyway, it just doesn't sound like a good idea to me. >> It sounds like >> omething that was meant for a clustering filesystem >> of some kind, not >> particularly for ZFS. >> >> If you are adding log devices to this, I have a >> couple of things to say: >> >> The whole point of a log device is to accelerate sync >> writes, by providing >> nonvolatile storage which is faster than the primary >> storage. You're not >> going to get this if any part of the log device is at >> the other side of a >> WAN. So either add a mirror of log devices locally >> and not across the WAN, >> or don't do it at all. >> >> >>> I am considering building a separate mirrored zpool >> of Flash disk that >>> span the frames, then creating zvols to use as log >> devices for the >>> data zpool. Will this work? Any other >> suggestions? >> >> This also sounds nonsensical to me. If your primary >> pool devices are Flash, >> then there's no point to add separate log devices. >> Unless you have another >> ype of even faster nonvolatile storage. > > Both frames are FC connected with Flash devices in the frame. Latencies are > additive, so there is benefit to a logging device. The cluster is a standard > HA cluster about 10km apart with identical storage in both locations, > mirrored using ZFS. There are quite a few metro clusters in the world today. Many use traditional mirroring software. Some use array-based sync replication. A ZFS-based solution works and behaves similarly. > Think about the potential problems if I don't mirror the log devices across > the WAN. If you use log devices, mirror them. -- richard -- Richard Elling rich...@nexenta.com +1-760-896-4422 ZFS and NexentaStor training, Rotterdam, July 13-15, 2010 http://nexenta-rotterdam.eventbrite.com/ _______________________________________________ zfs-discuss mailing list zfs-discuss@opensolaris.org http://mail.opensolaris.org/mailman/listinfo/zfs-discuss
