On 6/23/07, Erik Trimble <[EMAIL PROTECTED]> wrote:
Matthew Ahrens wrote: Basically, the descriptions of Copy on Write. Or does this apply only to Snapshots? My original understanding was that CoW applied whenever you were making a duplicate of an existing file.
CoW happens all the time. If you overwrite a file, instead of writing it to the same location on disk, ZFS allocates a new block, writes to that, and then creates a new tree in parallel (all on new, previously unused blocks). Then it changes the root of the tree to point to the newly allocated blocks.
Now that I think about it, I'm not sure that I can see any way to change the behavior of POSIX calls to allow for this type of mechanism. You'd effectively have to create a whole new system call with multiple file arguments. <sigh>
Files that are mostly the same, or exactly the same? If they're exactly the same, it's called a hardlink ;) If they're mostly the same, I guess, you could come up with a combination of a sparse file and a symlink. But I don't think the needed functionality is commonly enough used to bother implementing in kernel space. If you really want it in your application, do it yourself. Make a little file with two filenames, and a bitmap indicating which of them the application blocks should come from.
Now, wouldn't it be nice to have syscalls which would implement "cp" and "mv", thus abstracting it away from the userland app?
Not really. Different apps want different behavior in their copying, so you'd have to expose a whole lot of things - how much of the copy has completed? how fast is it going? - even if they never get used by the userspace app. And it duplicates functionality - you can do everything necessary in userspace with stat(), read(), write() and friends. _______________________________________________ zfs-discuss mailing list zfs-discuss@opensolaris.org http://mail.opensolaris.org/mailman/listinfo/zfs-discuss
