Re: Parallel Seq Scan

Jeff Janes <jeff(dot)janes(at)gmail(dot)com> writes:
> On Tue, Jan 27, 2015 at 11:08 PM, Heikki Linnakangas <
> hlinnakangas(at)vmware(dot)com> wrote:
>> OTOH, spreading the I/O across multiple files is not a good thing, if you
>> don't have a RAID setup like that. With a single spindle, you'll just
>> induce more seeks.
>>
>> Perhaps the OS is smart enough to read in large-enough chunks that the
>> occasional seek doesn't hurt much. But then again, why isn't the OS smart
>> enough to read in large-enough chunks to take advantage of the RAID even
>> when you read just a single file?

> In my experience with RAID, it is smart enough to take advantage of that.
> If the raid controller detects a sequential access pattern read, it
> initiates a read ahead on each disk to pre-position the data it will need
> (or at least, the behavior I observe is as-if it did that). But maybe if
> the sequential read is a bunch of "random" reads from different processes
> which just happen to add up to sequential, that confuses the algorithm?

If seqscan detection is being done at the level of the RAID controller,
I rather imagine that the controller would not know which process had
initiated which read anyway. But if it's being done at the level of the
kernel, it's a whole nother thing, and I bet it *would* matter.

regards, tom lane