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

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Message-ID: <20070305114945.GB454@lazybastard.org>
Date:	Mon, 5 Mar 2007 12:49:45 +0100
From:	Jörn Engel <joern@...ybastard.org>
To:	Anton Altaparmakov <aia21@....ac.uk>
Cc:	Ulrich Drepper <drepper@...hat.com>, 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()

On Mon, 5 March 2007 00:32:14 +0000, Anton Altaparmakov wrote:
> 
> I don't know how your compression algorithm works [...]

LogFS is designed for flash media, so it does not have to worry much
about reducing disk seeks.  It is log-structured, which simplifies
compression further.

When writing a block, it basically compresses it and appends it to the
log.  Writes only have to be byte-aligned, so no space is lost for
padding.

The bad news for posix_fallocate() is that even if libc is smart enough
to write random data, mmap() can still cause problems.  If the VM
decides to write a given page twice, the second write compresses better
and the medium has filled up between the two writes, the users will have
fun.

Jörn

-- 
Joern's library part 9:
http://www.scl.ameslab.gov/Publications/Gus/TwelveWays.html
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