linux-kernel - Re: The ext3 way of journalling

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Message-ID: <22619.1199865353@turing-police.cc.vt.edu>
Date:	Wed, 09 Jan 2008 02:55:53 -0500
From:	Valdis.Kletnieks@...edu
To:	Kyle Moffett <mrmacman_g4@....com>
Cc:	Andi Kleen <andi@...stfloor.org>, Theodore Tso <tytso@....edu>,
	LKML Kernel <linux-kernel@...r.kernel.org>
Subject: Re: The ext3 way of journalling

On Tue, 08 Jan 2008 22:21:02 EST, Kyle Moffett said:

> lvcreate -s -n "${VOLUME}-snap" "${VG}/${VOLUME}"

> Basically you can fsck the offline snapshot in the background.

Something the lvcreate manpage is specifically not clear about is:

Does this create a snapshot of the *disk* at that moment, or does it capture
"disk plus still-to-be-written blocks in the cache"? (Phrased differently, does
it Do The Right Thing regarding "blocks queued before lvcreate" and "blocks
queued for write after lvcreate")?

If the snapshot doesn't capture the blocks queued but still unwritten by
kjournald and similar, then you're still hitting the same old problems that
you always get when you fsck an "active disk".

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