On Sun, Jan 06, 2008 at 08:05:56AM -0800, sudarshan sridhar wrote: > My exact doubt is, if COW is default behavior of ZFS then does COWd > data written to the same physical drive where the filesystem > resides?
Just to clarify: there is no way to disable COW in ZFS. > If so the physical device capacity should be more that what the file > system size is. > > I mean in normal filesystem sinario, a partition with 1Gb with some > some filesystem (say ext2fs) is created, then use can save upto 1Gb > data under that. > > Is the same behavior with ZFS?. Because I feel since COW is default > ZFS require > 1Gb for one fileystem inorder to store COWed data. > > Please correct me if i am wrong. If you are trying to update as much data as you already have on disk, all in the same transaction, then yes, COW doubles your transient storage requirements for completing the transaction. Of course, noone ever updates (replaces) terabytes of data in one transaction. That's because the necessary bandwidth does not exist. So the amount of free space required in order to complete any given transaction is going to be a small fraction of the total amount of space in the given volume. You will only notice that you even need to have that much space available when your volume is very close to full. COW or no COW, if you're close to "volume full" you have a problem -- you are very likely to reach "volume full" and not having COW wouldn't save you. If you're trying to say that COW is inefficient, space-wise, what you'll find is that the space overhead for COW is in the noise for any large volume. Nico -- _______________________________________________ zfs-discuss mailing list zfs-discuss@opensolaris.org http://mail.opensolaris.org/mailman/listinfo/zfs-discuss
