Re: Parallel Seq Scan

On Thu, Jan 29, 2015 at 11:40 AM, Tom Lane <tgl@sss.pgh.pa.us> wrote:
>> 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.

That was my feeling too.  On the machine that Amit and I have been
using for testing, we can't find any really convincing evidence that
it matters.  I won't be a bit surprised if there are other systems
where it does matter, but I don't know how to find them except to
encourage other people to help test.

-- 
Robert Haas
EnterpriseDB: http://www.enterprisedb.com
The Enterprise PostgreSQL Company