From: VDA <VDA@port.imtp.ilyichevsk.odessa.ua>
To: John Ripley <jripley@riohome.com>
Cc: linux-kernel@vger.kernel.org
Subject: Re: COW fs (Re: Editing-in-place of a large file)
Date: Mon, 10 Sep 2001 12:28:51 +0300 [thread overview]
Message-ID: <8015001541.20010910122851@port.imtp.ilyichevsk.odessa.ua> (raw)
In-Reply-To: <3B9B80E2.C9D5B947@riohome.com>
In-Reply-To: <20010902152137.L23180@draal.physics.wisc.edu> <318476047.20010903002818@port.imtp.ilyichevsk.odessa.ua> <3B9B80E2.C9D5B947@riohome.com>
JR> I've tried this idea. I did an MD5 of every block (4KB) in a partition
JR> and counted the number of blocks with the same hash. Only about 5-10% of
JR> blocks on several filesystem were actually duplicates. This might be
JR> better if you reduced the block size to 512 bytes, but there's a
JR> question of how much extra space filesystem structures would then take
JR> up.
JR> Basically, it didn't look like compressing duplicate blocks would
JR> actually be worth the extra structures or CPU.
JR> On the other hand, a COW fs would be excellent for making file copying
JR> much quicker. You can do things like copying the linux kernel tree using
JR> 'cp -lR', but the files do not act as if they are unique copies - and
JR> I've been bitten many times when I forgot this. If you had COW, you
JR> could just copy the entire tree and forget about the fact they're
JR> linked.
Yeah, I'm mostly thinking about this kind of COW fs usage. You may copy
gigabytes in the instant and don't bother about tracking duplicate
files ("zero blocks left??? where's the hell I copied that .mpg's???").
Now, sometimes we use hardlinks as "poor man's COW fs", but
I bet it's error prone. Every now and then you forget it's a
hardlinked kernel tree and start happily hacking in it... :-(
A "compressor" which hunts and merges duplicate blocks is a bonus,
not a primary tool.
--
Best regards,
VDA
mailto:VDA@port.imtp.ilyichevsk.odessa.ua
http://port.imtp.ilyichevsk.odessa.ua/vda/
next prev parent reply other threads:[~2001-09-10 9:30 UTC|newest]
Thread overview: 29+ messages / expand[flat|nested] mbox.gz Atom feed top
2001-09-02 20:21 Editing-in-place of a large file Bob McElrath
2001-09-02 21:28 ` COW fs (Re: Editing-in-place of a large file) VDA
2001-09-09 14:46 ` John Ripley
2001-09-09 16:30 ` John Ripley
2001-09-10 2:43 ` Daniel Phillips
2001-09-10 2:58 ` David Lang
2001-09-09 17:41 ` Xavier Bestel
2001-09-10 1:29 ` John Ripley
2001-09-10 6:45 ` Ragnar Kjørstad
2001-09-14 10:06 ` Pavel Machek
2001-09-10 11:11 ` Ihar Filipau
2001-09-10 16:10 ` Kari Hurtta
2001-09-14 10:03 ` Pavel Machek
2001-09-10 9:28 ` VDA [this message]
2001-09-10 9:35 ` John P. Looney
2001-09-02 21:30 ` Editing-in-place of a large file Ingo Oeser
2001-09-03 0:59 ` Larry McVoy
2001-09-03 1:24 ` Ingo Oeser
2001-09-03 1:31 ` Alan Cox
2001-09-03 1:50 ` Ingo Oeser
2001-09-03 10:48 ` Alan Cox
2001-09-03 14:31 ` Daniel Phillips
2001-09-03 14:46 ` Bob McElrath
2001-09-03 14:54 ` Alan Cox
2001-09-03 15:42 ` Doug McNaught
2001-09-03 15:11 ` Richard Guenther
2001-09-03 21:19 ` Ben Ford
2001-09-03 4:27 ` Bob McElrath
2001-09-03 1:30 ` Daniel Phillips
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