Postgres Pro
Downloads
Home   >   mailing lists

Thread: Needs Suggestion

Needs Suggestion

From
SUBHAM ROY
Date:
My output of explain (analyze,buffers) is something like this:

shared hit=3796624 read=46038

So what is meant by read here? Does it indicates number of disk reads?

Does shared hit takes into account only the hit in the pg_buffercache of the postgres or it also takes into account the linux buffers that postgres uses?

--
Thank You,
Subham Roy,
CSE IIT Bombay.

Re: Needs Suggestion

From
Andreas Kretschmer
Date:
SUBHAM ROY <subham.iem@gmail.com> wrote:

> My output of explain (analyze,buffers) is something like this:
>
> shared hit=3796624 read=46038
>
> So what is meant by read here? Does it indicates number of disk reads?
>
> Does shared hit takes into account only the hit in the pg_buffercache of the
> postgres or it also takes into account the linux buffers that postgres uses?

shared hit: read from shared mem
shared read: read from disk into shared mem


Andreas
--
Really, I'm not out to destroy Microsoft. That will just be a completely
unintentional side effect.                              (Linus Torvalds)
"If I was god, I would recompile penguin with --enable-fly."   (unknown)
Kaufbach, Saxony, Germany, Europe.              N 51.05082°, E 13.56889°

Re: Needs Suggestion

From
Guillaume Lelarge
Date:
On 05/04/2011 08:39 AM, Andreas Kretschmer wrote:
> SUBHAM ROY <subham.iem@gmail.com> wrote:
>
>> My output of explain (analyze,buffers) is something like this:
>>
>> shared hit=3796624 read=46038
>>
>> So what is meant by read here? Does it indicates number of disk reads?
>>
>> Does shared hit takes into account only the hit in the pg_buffercache of the
>> postgres or it also takes into account the linux buffers that postgres uses?
>
> shared hit: read from shared mem
> shared read: read from disk into shared mem
>

To be more precise:

shared hit

  number of blocks directly read from PostgreSQL shared memory

read

  number of blocks PostgreSQL asked the OS to give him (then the OS may
  read the blocks from its own shared memory or may ask the disk
  subsystem to give him (and then the disk subsystem may read it from
  its own shared memory (SAN cache for example) or read it from the
  disk))


To put it more simply:

shared hit:
  number of blocks directly read from PostgreSQL shared memory

read:
  number of blocks read outside of PostgreSQL shared memory (meaning it
  may need disk read)


--
Guillaume
 http://www.postgresql.fr
 http://dalibo.com