Re: WAL prefetch

On Tue, Jun 19, 2018 at 4:04 PM Tomas Vondra <tomas(dot)vondra(at)2ndquadrant(dot)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=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.

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