On 19.06.2018 16:57, Ants Aasma wrote:
Right. My point is that while spawning bgworkers probably helps, I don't
expect it to be enough to fill the I/O queues on modern storage systems.
Even if you start say 16 prefetch bgworkers, that's not going to be
enough for large arrays or SSDs. Those typically need way more than 16
requests in the queue.
Consider for example [1] from 2014 where Merlin reported how S3500
(Intel SATA SSD) behaves with different effective_io_concurrency values:
[1]
https://www.postgresql.org/message-id/CAHyXU0yiVvfQAnR9cyH=HWh1WbLRsioe=mzRJTHwtr=2azsTdQ@mail.gmail.com
Clearly, you need to prefetch 32/64 blocks or so. Consider you may have
multiple such devices in a single RAID array, and that this device is
from 2014 (and newer flash devices likely need even deeper queues).'
For reference, a typical datacenter SSD needs a queue depth of 128 to saturate a single device. [1] Multiply that appropriately for RAID arrays.So
How it is related with results for S3500 where this is almost now performance improvement for effective_io_concurrency >8?
Starting 128 or more workers for performing prefetch is definitely not acceptable...
--
Konstantin Knizhnik
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The Russian Postgres Company