Date: Tue, 29 Dec 2009 12:22:11 +0100 From: Patrick Proniewski <patpro@patpro.net> To: "Ronald Klop" <ronald-freebsd8@klop.yi.org> Cc: freebsd-fs@freebsd.org Subject: Re: snapshot implementation Message-ID: <AAACDCE5-13C2-4F99-96A0-D274100EE28B@patpro.net> In-Reply-To: <op.u5o4f4px8527sy@82-170-177-25.ip.telfort.nl> References: <32CA2B73-3412-49DD-9401-4773CC73BED0@patpro.net> <alpine.GSO.2.01.0912231031450.1586@freddy.simplesystems.org> <4B3283F2.7060804@barryp.org> <3ea87f5f62bb8ba30d798d4605a64c83@localhost> <op.u5o4f4px8527sy@82-170-177-25.ip.telfort.nl>
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On 29 déc. 2009, at 11:51, Ronald Klop wrote: >>> That's certainly not the case with UFS snapshots, which can take a >>> long >>> 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 blocks. This is O(n) on the size of the FS and pauses the FS > during the 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 > readonly to create a snapshot. This is O(1). That's a very interesting precision. Thank you. patpro
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