Re: -F option, RAM usage, more...

On Wed, 4 Oct 2000, Mike Biamonte wrote:

> I understand from the docs that -F ".. prevents fsync()'s from
> flushing to disk after every transaction.." and that this boosts
> performance because RAM accesses are far faster than disk accesses.  I
> have also seen some impressive stats regarding the degree of this
> performance boost.

Normally, in order to ensure integrity of the database/datafiles,
PostgreSQL calls fsync() after each transaction - what this does, is it
ensures that all disk buffers are flushed to disk - ensuring that any
changes done in that transaction are committed to disk.

> 1. How often DOES PG flush to disk - if at all - when the -F option is
> invoked?  Can this be controllled?

Once after each transaction.

> 2. I have no first hand experience with Oracle, but I understand that
> one of the keys to its speed is its ability to pull the entire
> database (or selected tables) into RAM and work them from there.  Is
> this comparable to Postgres' -F option?

No. As far as I know PostgreSQL can't load an entire table into memory
beyond what could/would/might be cached in-memory as part of the OS'es
disk caching.

> 3. With -F, does PG pull the database into RAM at startup?  Or does it
> pull data into RAM as it is accessed.  (eg: the first few queries
> would be slower, but subsequent queries on same data would be
> faster...)?

No.

> 4. Does the -F option speed SELECTs as well as it speeds INSERTs?

I'll wager to assume that fsync() is only called after
INSERT/UPDATE/DELETE/ALTER's - since these are the only ones that modify
the on-disk data - although I could be wrong. So no, it wouldn't speed up
SELECT's.

> 5. I have a dedicated Linux 2.2.16 db server with 2GB of RAM. How can
> I be sure that Postgres is using all the RAM that it can? (-S
> option? -B option?)

Adjust your -S and -B options appropriately - not that I have any
"recommended values" :(



--
Dominic J. Eidson
                                        "Baruk Khazad! Khazad ai-menu!" - Gimli
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