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

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Date:	Fri, 16 Sep 2011 14:39:50 -0300
From:	Alvaro Herrera <alvherre@...mandprompt.com>
To:	Andres Freund <andres@...razel.de>
Cc:	Matthew Wilcox <matthew@....cx>, Andi Kleen <andi@...stfloor.org>,
	viro <viro@...iv.linux.org.uk>,
	linux-fsdevel <linux-fsdevel@...r.kernel.org>,
	linux-kernel <linux-kernel@...r.kernel.org>,
	robertmhaas <robertmhaas@...il.com>,
	Pg Hackers <pgsql-hackers@...tgresql.org>
Subject: Re: [HACKERS] Improve lseek scalability v3


Excerpts from Andres Freund's message of vie sep 16 14:27:33 -0300 2011:
> Hi,
> On Friday 16 Sep 2011 17:36:20 Matthew Wilcox wrote:

> > Does the query planner need to know the exact number of bytes in the file,
> > or is it after an order-of-magnitude?  Or to-the-nearest-gigabyte?
> It depends on where the information is used. For some of the uses it needs to 
> be exact (the assumed size is rechecked after acquiring a lock preventing 
> extension) at other places I guess it would be ok if the accuracy got lower 
> with bigger files (those files won't ever get bigger than 1GB).

One other thing we're interested in is portability.  I mean, even if
Linux were to introduce a new hypothetical syscall that was able to
return the file size at a ridiculously low cost, we probably wouldn't
use it because it'd be Linux-specific.  So an improvement of lseek()
seems to be the best option.

-- 
Álvaro Herrera <alvherre@...mandprompt.com>
The PostgreSQL Company - Command Prompt, Inc.
PostgreSQL Replication, Consulting, Custom Development, 24x7 support
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