Date: Thu, 20 Oct 2005 16:51:06 -0400 (EDT) From: user <user@dhp.com> To: Gayn Winters <gayn.winters@bristolsystems.com> Cc: "'Andrew P.'" <infofarmer@gmail.com>, freebsd-questions@freebsd.org Subject: RE: FreeBSD UFS2 snapshots, and math ... - resolved, but two more Qs Message-ID: <Pine.LNX.4.21.0510201642030.8180-100000@shell.dhp.com> In-Reply-To: <003201c5d5b4$753f9d20$c901a8c0@workdog>
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Folks, On Thu, 20 Oct 2005, Gayn Winters wrote: > > Imagine that each data block is marked with labels > > on change. It doesn't matter how many labels there > > are, there will be only one data block saved. > > In trying to follow this thread, I started looking around for a precise > definition of snapshot. > Man mksnap_ffs > wasn't too helpful, and googling for "snapshot" etc. wasn't fruitful. > I'm guessing that the original author of the thread (user at dhp.com) > may also need such a definition. Can someone provide a pointer to a > specification or at least an RFC-like paper? I found one: http://www.freebsd.org/cgi/cvsweb.cgi/src/sys/ufs/ffs/README.snapshot?rev=1.4 and further, I did some tests and discovered that what I was being told (by you folks) was indeed correct. No matter how many snapshots you have, the changes in blocks since the tiem before the first snapshot is only recorded in one of them. That is to say, if I do the following: - create 4 1gig /dev/zero filled files - create a snapshot - overwrite one of those 1gig files with /dev/random My free space will have decreased by 1gig. So far so good. If I then: - create a second snapshot - overwrite a different 1gig file with /dev/random My free space merely decreases by another 1gig. It makes sense to me now because it has occurred to me that since the second file had not changed between the creation of the first and second snapshot, there is no reason for _both_ snapshots to _both_ say "this 1gig random file used to be filled with zeros" - it would be redundant. So that's great ... but I am curious, how do they know ? I think my previous assumption (that the first _and_ the second snapshot file would _both_ have to record the change of file #2 from zero to random) was based on the notion that these snapshot files were totally autonomous and independent, and had no general organization behind them. If that was the case, then I am still fairly certain both snapshots would need to record the change of the second file. So what is the behind the scenes organization that makes it possible for the snapshot files to not duplicate data like that ? ALSO, I have noticed that if you: - dd 1gig /dev/zero file - create snapshot - overwrite that 1gig file with /dev/random (free space decreases by 1gig, as expected) - rewrite that 1gig file with /dev/zero again You _don't_ get that 1gig of free space back ... which surprises me, since it was all zeros before, and its all zeros now ... how does the snapshot know those are "different zeros" ? And what ramifications does this have for restoring, etc., if identical files do not get counted as identical in the snapshot ? thanks.
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