"Tom Lane" <tgl@sss.pgh.pa.us> writes:
> Gregory Stark <stark@enterprisedb.com> writes:
>> "Manoel Henrique" <mhenriquesgbd@gmail.com> writes:
>>> Yes, I'm relying on the assumption that backwards scan has the same cost as
>>> forward scan, why shouldn't it?
>
>> Because hard drives only spin one direction
>
> Good joke, but to be serious: we expect that forward scans will result
> in the kernel doing read-ahead, which will allow overlapping of
> CPU work to process one page with the I/O to bring in the next page.
Well it wasn't a joke but you're right that it's not the whole picture. But
then neither is considering interleaving of I/O with CPU work.
Hard drives spin in a particular direction which means if you stream I/O
requests to them in that direction you can stream data off the hard drive as
fast as it passes under the read head. That's going to be 50-60MB/s for a
single modern 7200rpm drive.
On the other hand if you send an I/O request for the previous block then you
have to wait a whole rotation before it passes under the head. On a 7200rpm
drive that's over 8ms which is a *lot* of CPU work to interleave. The most
bandwidth you'll be able to get is under 1MB/s.
However there's another reason fadvise helps -- the kernel or the drive gets a
chance to reorder the I/O. If we read-ahead a whole track's worth of I/O
backwards then during the first 8ms latency the kernel has a chance to notice
that it should reorder the queued up requests and do them the right way
around.
-- Gregory Stark EnterpriseDB http://www.enterprisedb.com Ask me about EnterpriseDB's 24x7 Postgres support!
