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Date: Sun, 18 Jan 2004 23:29:14 -0500 (EST)
From: Garrett Wollman <wollman@khavrinen.lcs.mit.edu>
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Subject: filesystem snapshot question
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<<On Sun, 18 Jan 2004 23:11:51 -0500, Jesse Guardiani <jesse@wingnet.net> said:

> So when a snapshot is active on a filesystem, does
> every disk write happen twice? Once to the real
> filesystem and once to the snapshot with old data?

No.  Writes only happen once.  If a block is part of a snapshot, a new
block on the disk is allocated to hold the updated data.

> Does a snapshot physically grow on the disk as
> changes are made to the real filesystem?

No, but the active filesystem shrinks, as blocks that were formerly
shared with the snapshot become exclusively the property of the
snapshot(s) which contain them.

-GAWollman