On Thu, Jan 29, 2015 at 11:40 AM, Tom Lane <t...@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 -- Sent via pgsql-hackers mailing list (pgsql-hackers@postgresql.org) To make changes to your subscription: http://www.postgresql.org/mailpref/pgsql-hackers
