Date: Tue, 29 Dec 2009 11:51:30 +0100 From: "Ronald Klop" <ronald-freebsd8@klop.yi.org> To: patpro <patpro@patpro.net>, "Barry Pederson" <bp@barryp.org> Cc: freebsd-fs@freebsd.org Subject: Re: snapshot implementation Message-ID: <op.u5o4f4px8527sy@82-170-177-25.ip.telfort.nl> In-Reply-To: <3ea87f5f62bb8ba30d798d4605a64c83@localhost> References: <32CA2B73-3412-49DD-9401-4773CC73BED0@patpro.net> <alpine.GSO.2.01.0912231031450.1586@freddy.simplesystems.org> <4B3283F2.7060804@barryp.org> <3ea87f5f62bb8ba30d798d4605a64c83@localhost>
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On Fri, 25 Dec 2009 15:29:53 +0100, patpro <patpro@patpro.net> wrote: > > On Wed, 23 Dec 2009 14:56:18 -0600, Barry Pederson <bp@barryp.org> wrot= e: >> "...there's virtually no overhead at all due to the copy-on-write >> architecture. In fact, sometimes it is faster to take a snapshot rathe= r >> than free the blocks containing the old data!" >> >> That's certainly not the case with UFS snapshots, which can take a lon= g >> time to complete (we're talking freezing your machine's disk activity >> for many minutes), and are limited to 20 total. > > > UFS uses copy on write. But you say many minutes to complete? Don't you > speak about dump(1), that uses snapshot as a basis to dump a live file > system? > I agree, UFS snapshot creation is not lightning-fast, but many minutes > seems a lot to me, and I never experienced such a long creation time. As far as I know UFS snapshots need to create a list of currently in use = =20 blocks. This is O(n) on the size of the FS and pauses the FS during the =20 snapshot. On large FS's this can take a long time. ZFS always maintains this list so it only needs to mark this list as =20 readonly to create a snapshot. This is O(1). Ronald.
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