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

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Date:	Sun, 04 Mar 2007 14:38:13 -0800
From:	Ulrich Drepper <drepper@...hat.com>
To:	Anton Altaparmakov <aia21@....ac.uk>
CC:	Arnd Bergmann <arnd@...db.de>,
	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()

Anton Altaparmakov wrote:
> And that is it.  No zeroing needs to happen at all because we
> have not updated the initialized size of the inode!

When you do it like this, who can the kernel/filesystem *guarantee* that
when the data is written there actually is room on the harddrive?

What you described seems like using truncate/ftruncate to increase the
file's size.  That is not at all what posix_fallocate is for.
posix_fallocate must make sure that the requested blocks on the disk are
reserved (allocated) for the file's use and that at no point in the
future will, say, a msync() fail because a mmap(MAP_SHARED) page has
been written to.

-- 
➧ Ulrich Drepper ➧ Red Hat, Inc. ➧ 444 Castro St ➧ Mountain View, CA ❖

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