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Re: File IO - why does PG do things in pages?

From: "Florian G(dot) Pflug" <fgp(at)phlo(dot)org>
To: Dan Eloff <dan(dot)eloff(at)gmail(dot)com>
Cc: pgsql-hackers(at)postgresql(dot)org
Subject: Re: File IO - why does PG do things in pages?
Date: 2009-11-26 22:37:05
Message-ID: 4B0F0311.6070807@phlo.org (view raw or flat)
Thread:
Lists: pgsql-hackers
Dan Eloff wrote:
> At the lower levels in PG, reading from the disk into cache, and 
> writing from the cache to the disk is always done in pages.
> 
> Why does PG work this way? Is it any slower to write whole pages 
> rather than just the region of the page that changed? Conversely, is 
> it faster? From what I think I know of operating systems, reading 
> should bring the whole page into the os buffers anyway, so reading
> the whole page instead of just part of it isn't much more expensive. 
> Perhaps writing works similarly?

First, data fetched from the disk is (except for data in temporary
tables, I believe) not stored in private memory of the backend process
doing the read(), but instead a a shared memory segment accessible by
all backend processes. This allows two different backend processes to
work modify the data concurrently without them stepping on each other's
toes. Note that immediatly writing back any changes is *not* an option,
since WAL logging mandates that all changes got to the WAL *first*.
Hence, if you were to write out each changed tuple immediately, you'd
have to first write the changes to the WAL *and* fsync the WAL to
guarantee they hit the disk first.

Sharing the data between backend processes requires a fair amount of
infrastructure. You need a way to locate a given chunk of on-disk data
in the shared memory buffer cache, and be able to acquire and release
locks on those buffers to prevent two backends from wrecking havoc when
they try to update the same piece of information. Organizing data in
fixed-sized chunks (which is what pages are) helps with keeping the
complexity of that infrastructure manageable, and the overhead
reasonably low.

There are also things like tracking the free space in a data file, which
also gets easier if you only have to track it page-wise (Is there free
space on this page or not), instead of having to track arbitrary ranges
of free space.

Finally, since data writes happen in units of blocks (and not bytes),
you need to guarantee that you do your IO in some multiple of that unit
anyway, otherwise you'd have a very hard time guaranteeing data
consistency after a crash. Google for "torn page writes", that should
give you more details about this problem.

Note, however, that a postgres page (usually 8K) is usually larger than
the filesystem's blocksize (usually 512b). So always reading in full
pages induces *some* IO overhead. Just not that much - especially since
the blocks comprising a page are extremely likely to be arranges
consecutively on disk, so there is no extra seeking involved.

This, at least, are what I believe to be the main reasons for doing
things in units of pages - hope this helps at least somewhat.

best regards,
Florian Pflug

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