Re: internal vs. external snaphots

["J. Hannken-Illjes" <hannken%eis.cs.tu-bs.de@localhost>]

On Nov 11, 2013, at 5:36 PM, Edgar Fuß <ef%math.uni-bonn.de@localhost> wrote:

> Since I have been experiencing some mysterious file system corruption on a fs 
> that was fsck-clean before being mounted, I would like to periodically fsck 
> that fs while mounted.
> The problem is that fs is 6T large.
> 
> If I use an internal snapshot (fsck_ffs -X), the server stalls for about an 
> hour, then panics (wapbl_deregister_allocation: out of resources) and, upon 
> reboot, needs longer for replaying the journal than an fsck would take.
> 
> If I use an external snapshot (fsck_ffs -x /somewhere/else), the snapshot 
> only 
> takes a few seconds, but fsck_ffs complains about unreferenced files, missing 
> blocks in the free list and bad summary information. I have tested both -x 
> and 
> -X on a smaller fs, and -X runs fine while -x complains.
> 
> So why do internal and external snapshots behave so differently? Are they 
> just 
> two completely diffrent aproaches to the same problem or what's the reason 
> for 
> using what seems to be a totally different strategy when the backup file is 
> inside the fs itself?

Two completely different approaches to get a stable view of a file system:

- Internal (or -X) creates a copy of the file system and runs the equivalent
of unmount on this copy.  Therefore unlinked files disappear and bitmaps are
consistent.

- External (or -x) just saves original data to the backup before it gets
overwritten.  This snapshot is just a frozen image in time containing
unreferenced but open files.

Both snapshot types get created from a synced file system.

As internal snapshots need much more I/O they are much slower.  It helps
a bit to use large (64k) file system block size (bigger blocks, less I/O).

The snapshot type is determined from the location of the backup. 

> Is there any documentation on the subject?

Not yet, sorry.

--
J. Hannken-Illjes - hannken%eis.cs.tu-bs.de@localhost - TU Braunschweig 
(Germany)