There are better ways to snapshotting

Posted May 10, 2009 18:36 UTC (Sun) by anton (subscriber, #25547)
In reply to: There are better ways to snapshotting by butlerm
Parent article: The two sides of reflink()

One of the advantages of ZFS style snapshotting is it is a constant time operation that is far more efficient than walking a directory tree and creating thousands of new inodes. It doesn't work for making a (writable) snapshot of a snapshot, however

That may be a limitation of ZFS, but it's not a necessary limitation. LLFS can do it, and nothing I have read about Btrfs indicates that it cannot do it.

Copy-on-write filesystems could use a good interface (like this one) to make efficient *copies* of files in the same (extended) filesystem.

Yes, implementing cp more efficiently seems to be the main benefit I see from this system call.

There are better ways to snapshotting

Posted May 11, 2009 5:22 UTC (Mon) by butlerm (subscriber, #13312) [Link]

Netapp does writable snapshots of writable snapshots. The advantage of the
ZFS scheme is you can have an unlimited number of snapshots, where with
Netapp these days I believe you get 256.

The ZFS scheme is based on logical sequence numbers, Netapp uses block maps.
I understand that BTRFS allows nested writable snapshots, but that comes at a
general performance cost that ZFS doesn't have.