Len,
For some interesting background on ZIL options in a lower-end configuration,
with benchmarks, you could check out
<http://napp-it.org/doc/manuals/benchmarks.pdf>
though it targets OmniOS instead of SmartOS. (Spoiler: Intel DC3700 or ZeusRAM…)
Note that the ZIL is write-only unless you crash or lose power, in which case
its contents need to be replayed.
(The ZIL is a high-IOPS stable storage for a replay log of random sync writes,
which allows ZFS to cache the writes in RAM and flush them to the data drives
in an ordered fashion, reducing the random write IOPS load on the data drives,
which improves performance and could reduce wear on an SSD pool as well. The
contents of the ZIL remain in RAM until committed, so there is no need to read
the ZIL unless you crash.)
Therefore, write-endurance and power-loss protection are essential for a ZIL
device. OTOH, if the ZIL fails, the system should keep running, just with
reduced sync write performance. So you only need to mirror the ZIL if you can’t
tolerate a drop in performance for the time it takes to provision a new ZIL
device.
Best,
Chris
Am 09.05.2014 um 08:42 schrieb Len Weincier via smartos-discuss
<[email protected]>:
>>
>> To the original question, the parts we use at Joyent can always be found
>> at http://eng.joyent.com/manufacturing. Those are the ones with which
>> we have direct experience and in which we have the most confidence. We
>> always recommend using our Certified configurations. Notice that we
>> don't use L2ARC; demand for it in JPC has been limited to nonexistent (I
>> wonder if you're really sure you need it).
>>
>> I should also mention that the Mach16 is EOL and HGST/STEC have not come
>> out with a replacement for it (Keith's pet peeve #464: vendors who EOL
>> good hardware without providing a similar replacement). As such, you
>> may have difficulty sourcing that part. The S840z is a fine substitute
>> for it in the same form factor, and is much faster; however, it's also
>> much more expensive. HGST has unfortunately put me in a situation where
>> I am unable to recommend any of their products for use as a slog in the
>> same price range as the Mach16. Unless they come out with something
>> great later this year, we'll probably be stuck with the DCS3700.
>>
>>
>
> 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 ?
>
> 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 ?
>
> Thanks
> Len
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