"Wee Yeh Tan" <[EMAIL PROTECTED]> wrote: > On Wed, Feb 27, 2008 at 10:42 PM, Marcus Sundman <[EMAIL PROTECTED]> > wrote: > > Darren J Moffat <[EMAIL PROTECTED]> wrote: > > > Marcus Sundman wrote: > > > > Nicolas Williams <[EMAIL PROTECTED]> wrote: > > > >> On Wed, Feb 27, 2008 at 05:54:29AM +0200, Marcus Sundman > > > >> wrote: > > > >>> Nathan Kroenert <[EMAIL PROTECTED]> wrote: > > > >>>> Are you indicating that the filesystem know's or should > > > >>>> know what an application is doing?? > > > >>> Maybe "snapshot file whenever a write-filedescriptor is > > > >>> closed" or somesuch? > > > >> Again. Not enough. Some apps (many!) deal with multiple > > > >> files. > > > > > > > > So what? Why would every file-snapshot have to be a file that's > > > > valid for the application(s) using it? (Certainly zfs snapshots > > > > don't provide that property either, nor any other > > > > backup-related system I've seen.) > > > > > > If it isn't how does the user or application know that is safe > > > to use that file ? > > > > Unless the files contain some checksum or somesuch then I guess it > > doesn't know it's safe. However, that's unavoidable unless the > > application can use a transaction-supporting fs api. > > Checksums only tell you the data file is good. If you have a whole > load of backups (one every nano-second) and none of them have a good > checksum, you are still very screwed.
True. However, this is equally true for zfs snapshots. If I undestood the concept of CDP correctly then each zfs snapshot would provide a subset of the set of all versions in the CDP database. Thus, CDP couldn't possibly provide less protection than zfs snapshots (although it might be harder to find the right versions of files). So, if you think zfs snapshots provide enough protection then you can't claim CDP doesn't. > > > Is it okay to provide a snapshot of a file that is corrupt and > > > will cause further more serious data corruption in the > > > application ? > > > > Well, apparently so. That's what zfs snapshots do. That's what all > > backup tools do. Sure it would be better to have full transactions > > in the fs api, but without that I don't think it's possible to do > > any better than "the file might be corrupt or it might not, good > > luck if your file format doesn't support corruption-detection". > > A good backup practice increases (significantly) the likelihood of > getting a usable backup. E.g. you quiesce Oracle before you start > your backup to make sure that the datafiles you backup are consistent. True for both ZFS snapshots and CDP, except that with CDP you don't have to make the actual snapshot since that's automated. > Still, you are missing the point. What's the point of backing up if > you cannot use it for restoring your environment? I think you are missing the point if you think ZFS snapshots are capable of something CDP is not. Also, I though the author of the original message wasn't particularly interested in restoring the environment, but more about restoring individual files. As a kind of version history, or filesystem undo if you will. Maybe I misunderstood him. - Marcus _______________________________________________ zfs-discuss mailing list zfs-discuss@opensolaris.org http://mail.opensolaris.org/mailman/listinfo/zfs-discuss
