To: "Dawid Kuroczko" <qnex42@gmail.com>
Cc: "Chris Hoover" <revoohc@gmail.com>,
	"PGSQL Performance" <pgsql-performance@postgresql.org>
Subject: Re: PostgreSQL NetApp and NFS
In-Reply-To: <758d5e7f0803210234l785b115as6f4b42f42fa97f13@mail.gmail.com>
	(Dawid Kuroczko's message of "Fri\, 21 Mar 2008 10\:34\:09 +0100")
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References: <1d219a6f0803201232g54924fb3ka206c9b03e4664c7@mail.gmail.com>
	<758d5e7f0803210234l785b115as6f4b42f42fa97f13@mail.gmail.com>
From: Gregory Stark <stark@enterprisedb.com>
Organization: EnterpriseDB
Date: Fri, 21 Mar 2008 13:22:26 +0000
Message-ID: <87tzj0qkj1.fsf@oxford.xeocode.com>
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"Dawid Kuroczko" <qnex42@gmail.com> writes:

> It is also possible to present block devices from NetApp over iSCSI or FC
> (I am not sure about licensing model though).  You get all the goodies
> like thin provisioning (only non-zero blocks are allocated), snapshots and
> all, but you see it as a block device.  Works fine.

Note that Postgres doesn't expect to get "out of space" errors on writes
specifically because it pre-allocates blocks. So this "thin provisioning"
thing sounds kind of dangerous.

> It is also worth to mention that NetApp utilizes somewhat "copy on write"
> write strategy -- so whenever you modify a block, new version of the block
> is written on its WAFL filesystem.  In practical terms it is quite resilient to
> random writes (and that read performance is not stellar ;)).

Again, Postgres goes to some effort to keep its reads sequential. So this
sounds like it destroys that feature. Random writes don't matter so much
because Postgres has its WAL which it writes sequentially. Writes to data
files aren't in the critical path and can finish after the transaction is
committed.

-- 
  Gregory Stark
  EnterpriseDB          http://www.enterprisedb.com
  Get trained by Bruce Momjian - ask me about EnterpriseDB's PostgreSQL training!