On Fri, Sep 25, 2009 at 5:47 PM, Marion Hakanson <hakansom@xxxxxxxx> wrote:
> j@xxxxxxxxxxxx said:
>> For a predominantly NFS server purpose, it really looks like a case of the
>> slog has to outperform your main pool for continuous write speed as well as
>> an instant response time as the primary criterion. Which might as well be a
>> fast (or group of fast) SSDs or 15kRPM drives with some NVRAM in front of
> I wonder if you ran Richard Elling's "zilstat" while running your
> workload. That should tell you how much ZIL bandwidth is needed,
> and it would be interesting to see if its stats match with your
> other measurements of slog-device traffic.
Yes, but if it's on NFS you can just figure out the workload in MB/s
and use that as a rough guideline.
Problem is most SSD manufactures list sustained throughput with large
IO sizes, say 4MB, and not 128K, so it is tricky buying a good SSD
that can handle the throughput.
> I did some filebench and "tar extract over NFS" tests of J4400 (500GB,
> 7200RPM SATA drives), with and without slog, where slog was using the
> internal 2.5" 10kRPM SAS drives in an X4150. These drives were behind
> the standard Sun/Adaptec internal RAID controller, 256MB battery-backed
> cache memory, all on Solaris-10U7.
> We saw slight differences on filebench oltp profile, and a huge speedup
> for the "tar extract over NFS" tests with the slog present. Granted, the
> latter was with only one NFS client, so likely did not fill NVRAM. Pretty
> good results for a poor-person's slog, though:
I did a smiliar test with a 512MB BBU controller and saw no difference
with or without the SSD slog, so I didn't end up using it.
Does your BBU controller ignore the ZFS flushes?
> Just as an aside, and based on my experience as a user/admin of various
> NFS-server vendors, the old Prestoserve cards, and NetApp filers, seem
> to get very good improvements with relatively small amounts of NVRAM
> (128K, 1MB, 256MB, etc.). None of the filers I've seen have ever had
> tens of GB of NVRAM.
They don't hold on to the cache for a long time, just as long as it
takes to write it all to disk.
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