Subject: re: Which snapshot strategy to use? was: How to capture all file system writes
To: Eduardo Horvath <eeh@NetBSD.org>
From: matthew green <mrg@eterna.com.au>
List: tech-kern
Date: 10/24/2003 09:41:28
   
   > I can see two different approaches to file system snapshots:
   > 
   > 1) Kirk McKusick ffs snapshots from FreeBSD. It is an extension of the
   >    ffs file system with these properties (I hope I get them right..):
   > 
   >    - Backing store is always the snapshotted file system.
   >    - The snapshot appears as a file whose size is the size of the file
   >      system. Blocks unallocated at the time of the snapshot read as 0's.
   >    - It allows up to 20 snapshots per file system
   >    - The snapshots survive unmounts and reboots.
   > 
   >    - Mounting or dumping the snapshot is possible through vnd(4) and/or
   >      some modifications to dump(8).
   > 2) A somewhat generic approach similar to the fssnap from Solaris:
   > 
   >    - Backing store must be on another file system (or swap / memory).
   >    - The snapshot appears as a raw/block pseudo device. It is possible
   >      to freeze the unallocated blocks as well.
   >    - Number of snapshots is bound by the number of pseudo devices.
   >    - The snapshots get cleared on unmount.
   
   3) Use union mounts.  The lower level becomes the filesystem snapshot,
   whild the upper layer is the operating filesystem.


this doesn't work for already open file descriptors on the lower
layer does it?  you have to change the lower layer to be r/o and
some how migrate the fd's tothe upper layer, sounds really ugly.