linux-ext4 - Re: [RFC] Heads up on sys

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Message-ID: <20070306145009.GB3661@duck.suse.cz>
Date:	Tue, 6 Mar 2007 15:50:09 +0100
From:	Jan Kara <jack@...e.cz>
To:	Ulrich Drepper <drepper@...hat.com>
Cc:	Christoph Hellwig <hch@...radead.org>,
	Mingming Cao <cmm@...ibm.com>,
	Andrew Morton <akpm@...ux-foundation.org>, nscott@...nex.com,
	"Amit K. Arora" <aarora@...ux.vnet.ibm.com>,
	linux-fsdevel@...r.kernel.org, linux-kernel@...r.kernel.org,
	linux-ext4@...r.kernel.org, suparna@...ibm.com, alex@...sterfs.com,
	suzuki@...ibm.com
Subject: Re: [RFC] Heads up on sys_fallocate()

On Tue 06-03-07 06:36:09, Ulrich Drepper wrote:
> Christoph Hellwig wrote:
> > fallocate with the whence argument and flags is already quite complicated,
> > I'd rather have another call for placement decisions, that would
> > be called on an fd to do placement decissions for any further allocations
> > (prealloc, write, etc)
> 
> Yes, posix_fallocate shouldn't be made more complicated.  But I don't
> understand why requesting linear layout of the blocks should be an
> option.  It's always an advantage if the blocks requested this way are
> linear on disk.  So, the kernel should always do its best to make this
> happen, without needing an additional option.
  Actually, it's not that simple. You want linear layout of blocks you are
going to read. That is not necessary a linear layout of blocks in a single
file - trace sometime a start of some complicated app like KDE. You find
it's seeking like a hell because it needs a few blocks from a ton of
distinct files (shared libs, config files, etc). As these files are mostly
read only, it's advantageous to interleave them on disk or at least keep
them close.

									Honza
-- 
Jan Kara <jack@...e.cz>
SuSE CR Labs
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