On Tue, Jun 19, 2018 at 4:04 PM Tomas Vondra <tomas.von...@2ndquadrant.com> 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=2azs...@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. Regards, Ants Aasma [1] https://www.anandtech.com/show/12435/the-intel-ssd-dc-p4510-ssd-review-part-1-virtual-raid-on-cpu-vroc-scalability/3
