Jeff Janes <jeff.ja...@gmail.com> writes: > On Tue, Jan 27, 2015 at 11:08 PM, Heikki Linnakangas < > hlinnakan...@vmware.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 -- Sent via pgsql-hackers mailing list (pgsql-hackers@postgresql.org) To make changes to your subscription: http://www.postgresql.org/mailpref/pgsql-hackers