On Wed, Feb 27, 2008 at 01:45:41AM +0800, Uwe Dippel wrote: > Sorry, I don't understand any of this. But I never pretended I did.
Well, if you want some feature then you should understand what it is. Sure "continuous data protection" sounds real good, but you have to understand that any CDP solution has to have knowledge of, or even be driven by your applications -- otherwise CDP isn't really. This is explained below. > My post was on something else: > In principle we have three types of write; atomic view, please: "atomic view"? > 1. Create. The new file needs to be written only, no backup/CDP > needed; identical to any conventional system. > 2. Edit/Modify. Here we need to store some incremental/differential > file content. rsync-like, that is. The rub is this: how do you know when a file edit/modify has completed? The answer is: it depends on what application we're talking about! > 3. Remove. Also this is similar to the conventional system, except > that the files need to be retired and the blocks *not* be marked as > 'available'. If an application has many files then an "edit/modify" may include updates and/or removals of more than one file. So once again: how do you know when an edit/modify has completed? The answer is still the same. My point is this: because the interesting times at which to take checkpoints are application-specific, we can't have a useful application-independent CDP solution. An application-independent CDP solution would not necessarily (not likely!) produce checkpoints that are safe to restore to. If you don't know whether it's safe to restore to a given checkpoint, and finding out is "hard", then what use is that checkpoint? And if you know it isn't safe then the checkpoint is truly useless -- it'll just sit there, taking up space. CDP really must be an application feature. Using ZFS snapshots could certainly make it easier to implement app-level CDP, and having the ability to snapshot/clone at a finer granularity than datasets (e.g., per-file) would help too. But ZFS _alone_ cannot provide a useful CDP solution. Nico -- _______________________________________________ zfs-discuss mailing list zfs-discuss@opensolaris.org http://mail.opensolaris.org/mailman/listinfo/zfs-discuss
