Re: Index Scans become Seq Scans after VACUUM ANALYSE

On Wed, 24 Apr 2002, Michael Loftis wrote:

> A Block-sized read will not be rboken up. But if you're reading ina
> size bigger than the underlying systems block sizes then it can get
> broken up.

In which operating systems, and under what circumstances?

I'll agree that some OSs may not coalesce adjacent reads into a
single read, but even so, submitting a bunch of single reads for
consecutive blocks is going to be much, much faster than if other,
random I/O occured between those reads.

> If the underlying
> block size is 8KB and you dump 4MB down on it, the OS may (and in many
> cases does) decide to write part of it, do a read ona nearby sector,
> then write the rest. This happens when doing long writes that end up
> spanning block groups because the inodes must be allocated.

Um...we're talking about 64K vs 8K reads here, not 4 MB reads. I am
certainly not suggesting Posgres ever submit 4 MB read requests to the OS.

I agree that any single-chunk reads or writes that cause non-adjacent
disk blocks to be accessed may be broken up. But in my sense,
they're "broken up" anyway, in that you have no choice but to take
a performance hit.

> Further large read requests can of course be re-ordered by hardware.
> ...The OS also tags ICP, which can be re-ordered on block-sized chunks.

Right. All the more reason to read in larger chunks when we know what we
need in advance, because that will give the OS, controllers, etc. more
advance information, and let them do the reads more efficiently.

cjs
--
Curt Sampson <cjs(at)cynic(dot)net> +81 90 7737 2974 http://www.netbsd.org
Don't you know, in this new Dark Age, we're all light. --XTC