On 09/30/2015 03:45 PM, Patric Bechtel wrote:
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> Hi Tomas,
>
> Tomas Vondra schrieb am 30.09.2015 um 14:01:
>> Hi,
>>
...
>>
>> I've also done a few runs with compression, but that reduces the performance a bit
>> (understandably).
>
> I'm somewhat surprised by the influence of the rsize value. I will recheck that. In my case, the
> compression actually improved throughput quite a bit, but that might change depending on CPU speed
> vs IO speed. Our CPU's are quite powerful, but the SSD are just SATA Samsung/OCZ models at least
> 18 months old. Also, I measured the write performance over several hours, to push the internal gc
> of the SSD to its limits. We had some problems in the past with (e.g. Intel) SSD's and their
> behaviour (<1MB/s), so that's why I put some emphasis on that.
I think it really depends on how random the workload. If the workload is
random (as for example the workload simulated by pgbench), the
recordsize seems to matter a lot - perhaps your workload is not as
random? Same for compression.
I'm no ZFS expert, but I assume ARC tracks those records chunks, so
using 128kB records (default) means you have 16x less slots than with
8kB records. You may still cache the same amount of data, but the cache
may not adapt that well to your active set.
regards
--
Tomas Vondra http://www.2ndQuadrant.com
PostgreSQL Development, 24x7 Support, Remote DBA, Training & Services
