On Fri, May 09, 2014 at 08:42:51AM +0200, Len Weincier via smartos-discuss wrote:
> I see from the manufacturing docs that you have a single 50GB device > (270-022 usually). From reading around it seems to be recommend that > the slog be mirrored as it is taking the writes and if one fails there > is a spare. Do you find having only the one slog device is ok ? Yes. The slog is not "taking the writes"; it's being used to store the ZIL. If it fails on a running system, the ZIL will be written to the main pool; correctness will not be compromised. Of course, there are lots of ways for a device to fail. If you imagine the possibility that a device fails by taking 50x longer than normal to complete writes, your mirror looks a lot less useful (read: not at all). The only certainty is that buying 2 devices will cost you twice as much. The likelihood of the specific multiple-failure sequences that can lead to data loss here is extremely low. I personally have never seen it happen and consider it borderline contrived, much like the meteor strike and nuclear war scenarios. Given enough machines and enough time, I'm sure it will eventually happen (and when it does, it will be caused by operator error), but in the meantime I still have to think about cost. If your pool is being used to track the beneficial owners of trillions of dollars of Treasury debt, go ahead and spend the extra money. > Also what sort of performance change / increase could one expect from > using and slog device, we have the DCS3700’s ? I am trying to > understand the way the slog works and how much improvement I can > predictably expect from an SSD slog ? Any reasonable slog vs. no slog will provide a significant improvement (order of magnitude reduction in latency or more) for any workload doing synchronous writes. I have never attempted to come up with a model that can map the numbers in a particular slog's marketing brochure to delivered performance; instead, I get evaluation units and typically use a dozen or so filebench profiles along with DTrace to get a rough estimate of what delivered (i.e., filesystem) performance will look like and how the slog itself is behaving. That test is conducted on a pool with that particular slog and the exact pool layout we would use. This is not as good as observing a production workload, but given the breadth of what our customers do, it's all that's practical. If you have just a single machine, and know what your workload is, you can do better. As always, watch for things you can't explain and make sure you understand them before accepting the results. ------------------------------------------- smartos-discuss Archives: https://www.listbox.com/member/archive/184463/=now RSS Feed: https://www.listbox.com/member/archive/rss/184463/25769125-55cfbc00 Modify Your Subscription: https://www.listbox.com/member/?member_id=25769125&id_secret=25769125-7688e9fb Powered by Listbox: http://www.listbox.com
