linux-kernel - Re: [HACKERS] Improve lseek scalability v3

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Date:	Mon, 19 Sep 2011 07:25:00 -0600
From:	Matthew Wilcox <matthew@....cx>
To:	Stephen Frost <sfrost@...wman.net>
Cc:	Benjamin LaHaise <bcrl@...ck.org>,
	Andres Freund <andres@...razel.de>,
	Andi Kleen <andi@...stfloor.org>, viro@...iv.linux.org.uk,
	linux-fsdevel@...r.kernel.org, linux-kernel@...r.kernel.org,
	robertmhaas@...il.com, pgsql-hackers@...tgresql.org
Subject: Re: [HACKERS] Improve lseek scalability v3

On Mon, Sep 19, 2011 at 08:31:00AM -0400, Stephen Frost wrote:
> * Benjamin LaHaise (bcrl@...ck.org) wrote:
> > For such tables, can't Postgres track the size of the file internally?  I'm 
> > assuming it's keeping file descriptors open on the tables it manages, in 
> > which case when it writes to a file to extend it, the internally stored size 
> > could be updated.  Not making a syscall at all would scale far better than 
> > even a modified lseek() will perform.
> 
> We'd have to have it in shared memory and have a lock around it, it
> wouldn't be cheap at all.

Yep, that makes perfect sense.  After all, the kernel does basically the
same thing to maintain this information; why should we have userspace
duplicating the same infrastructure?

I must admit, I'd never heard of this usage of lseek to get the current
size of a file before; I'd assumed everybody used fstat.  Given this
legitimate reason for a high-frequency calling of lseek, I withdraw my
earlier objection to the patch series.

-- 
Matthew Wilcox				Intel Open Source Technology Centre
"Bill, look, we understand that you're interested in selling us this
operating system, but compare it to ours.  We can't possibly take such
a retrograde step."
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