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

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Message-Id: <200703050136.37181.arnd@arndb.de>
Date:	Mon, 5 Mar 2007 01:36:36 +0100
From:	Arnd Bergmann <arnd@...db.de>
To:	Jörn Engel <joern@...ybastard.org>
Cc:	Ulrich Drepper <drepper@...hat.com>,
	Anton Altaparmakov <aia21@....ac.uk>,
	Christoph Hellwig <hch@...radead.org>,
	Dave Kleikamp <shaggy@...ux.vnet.ibm.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	"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, cmm@...ibm.com,
	alex@...sterfs.com, suzuki@...ibm.com
Subject: Re: [RFC] Heads up on sys_fallocate()

On Monday 05 March 2007, Jörn Engel wrote:
> That actually causes an interesting problem for compressing filesystems.
> The space consumed by blocks depends on their contents and how well it
> compresses.  At the moment, the only option I see to support
> posix_fallocate for LogFS is to set an inode flag disabling compression,
> then allocate the blocks.
> 
> But if the file already contains large amounts of compressed data, I
> have a problem.  Disabling compression for a range within a file is not
> supported, so I can only return an error.  But which one?

Using the current glibc implementation on a compressed file system ideally
should be a very expensive no-op because you won't actually allocate much
space for a file when writing zeroes to it. You also don't benefit of a
contiguous allocation in logfs, since flash has uniform seek times over
all the medium.

I'd suggest you implement posix_fallocate as an real nop and just return
success without doing anything. You could also return ENOSPC in case
the blocks requested by posix_fallocate don't fit on the medium without
compression, but that is more or less just guesswork (like statfs is).

	Arnd <><
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