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Re: Raw devices vs. Filesystems

From: Christopher Browne <cbbrowne(at)acm(dot)org>
To: pgsql-admin(at)postgresql(dot)org
Subject: Re: Raw devices vs. Filesystems
Date: 2004-03-29 18:28:47
Message-ID: m3oeqf4ie8.fsf@wolfe.cbbrowne.com (view raw or flat)
Thread:
Lists: pgsql-admin
After takin a swig o' Arrakan spice grog, el_vigia_ec(at)hotmail(dot)com ("Jaime Casanova") belched out:
> Can you tell me (or at least guide me to a palce where i can find the
> answer) what are the benefits of filesystems over raw devices?

For PostgreSQL, filesystems have the merit that you can actually use
them.  PostgreSQL doesn't support use of "raw devices."

Two major benefits of using filesystems as opposed to raw devices are
that:

a) The use of raw devices is dramatically non-portable; you have to
   reimplement data access on every platform you are trying to
   support; 

b) The use of raw devices essentially mandates that you implement
   some form of generic filesystem on top of them, which adds
   considerable complexity to your code.

Two benefits to raw devices are claimed...

c) It's faster.  But that assumes that the "cooked" filesystems are
   implemented fairly badly.  That was typically true, a dozen
   years ago, but it isn't so typical now, particularly with a
   fancy cacheing controller.

d) It guarantees application control of update ordering.  Of course,
   with a cacheing controller, or disk drives that lie to one degree
   or another, those guarantees might be gone anyways.

There are other filesystem advantages, such as

e) Shifting "cooked" data around may be as simple as a "mv," whereas
   reorganizing on raw disk requires DB-specific tools...

> And what filesystem is the best for postgresql performance?

That would depend, assortedly, on what OS you are using, what kind of
hardware you are running on, what kind of usage patterns you have, as
well as on how you define the notion of "best."

Absent of any indication of any of those things, the best that can be
said is "that depends..."
-- 
(format nil "~S(at)~S" "cbbrowne" "acm.org")
http://cbbrowne.com/info/languages.html
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Your XGP output is startling.

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