[OmniOS-discuss] Pliant/Sandisk SSD ZIL

[Derek Yarnell]

On 2/17/14, 7:31 PM, Richard Elling wrote:
> On Feb 17, 2014, at 2:48 PM, Derek Yarnell <derek at umiacs.umd.edu> wrote:
> 
>> Hi,
>>
>> So we bought a new Dell R720xd with 2 Dell SLC SSDs which were shipped
>> as a Pliant-LB206S-D323-186.31GB via format.
> 
> Pliant SSDs (note: Pliant was purchased by Sandisk in 2011) are optimized for
> lots of concurrent I/O operations. This is not the kind of workload generated by
> the ZIL, which is more contiguous, single thread-like.

I realize that they may not be as good as Stec/HGST ZeusRAM drives for
the slog.  I still can't wrap my head around that it is as bad as having
no discrete slog.

> 
> Never ever use zpool iostat to measure application performance. zpool iostat
> measures workload to the vdevs, showing back-end operations to disk. As such
> there is no correlation to client-side operations of any sort, especially writes and
> metadata updates. You'll need to go up the stack and see what it is doing. For NFS
> I highly recommend nfssvrtop. To see the response time of the Pliant, use "iostat -x"
> or, as I prefer, "iostat -zxCn"

Yes I realize that iostat will show me this information and the svc_t
for the Pliant ssd(s) is anywhere from 2-7ms.  But zpool iostat will
show you your ZIL writes accurately no?  I realize that it will then
coalesce these into its transactions and write it out at the 5sec interval.

> 
> Note: response time (measured by iostat -x as a variant of "svc_t") is the critical 
> measurement for NFS workloads. Bandwidth is almost meaningless in analyzing
> NFS.

Yes and I have done this too.  The average RTT on untaring is 43.000 ms.
 I guess we will just be getting another set of ZeusRAM drives.

Thanks,
derek

-- 
Derek T. Yarnell
University of Maryland
Institute for Advanced Computer Studies