Re: kern/37425: fss_snapshot_mount panic during fsck

[Scott Ellis <scotte%warped.com@localhost>]

On 11/25/2008 6:20 AM, Juergen Hannken-Illjes wrote:
> The following reply was made to PR kern/37425; it has been noted by GNATS.
>
> From: Juergen Hannken-Illjes<hannken%eis.cs.tu-bs.de@localhost>
> To: gnats-bugs%netbsd.org@localhost
> Cc:
> Subject: Re: kern/37425: fss_snapshot_mount panic during fsck
> Date: Tue, 25 Nov 2008 15:15:53 +0100
>
>   On Mon, Nov 24, 2008 at 06:05:23PM -0800, Scott Ellis wrote:
>   [snip]
>   >  Script used to test is attached (yes, it's a hack, but it was quick to
>   >  write!) as pffstest.pl.  It requires the directory structure of
>   >  /.snapshot/.store to be created, and vnd 11-14 to be available.
>
>   How should the /.snapshot/.store hierarchy look like?

I believe /.snapshot/.store/ will get populated with snapshot files (5 
at a time?) with the filename of the date and time the snapshot was 
taken.  The script should automatically do that for you.

>   >          # Make a new store file
>   >          exec(`fssconfig -c fss0 / $newfile`);
>   >          exec(`fssconfig -u fss0`);
>   >
>   >          # Attach it to the vnode
>   >          exec(`vnconfig -r $vnd{""} $newfile`);
>
>   If I get this right you are trying to create a vnd device from the
>   snapshot backup.  If this succeeds, there is a bug in the vnd driver.
>   Snapshot backups cannot be opened, read or written -- they can only
>   be accessed through /dev/{r,}fss{n}.

It's been ages since I hacked together that script, but if memory 
serves, the general logic is supposed to be something like:

1) Create snapshot #1
2) Attach vnd11 to snapshot #1 (using 
"/.snapshot/.store/2008-11-25-whatever" as the vnd backing file)
3) Mount vnd11a (or whatever) as "/.snapshot/2008-11-25-whatever"
4) Create snapshot #2
5) Repeat steps 1-3 until vnd11-vnd14 are filled snapshots #1 through #4
6) umount and unconfigure vnd11/snapshot #1
7) Free the snapshot #1
8) Perform steps 1-3 (so that snapshot #1 is now the most recent snapshot)
9) Repeat steps 6-8 with the oldest snapshot, loop forever

The gist is that I tried to always have 4 snapshots configured and 
mounted (via vnd).  As soon as all 4 are used, the oldest one is removed 
and a new snapshot is taken.

In my quick test, creating /.snapshots/.store/ and running the script 
twice gave the panic (the first time through it just created the first 
.store/foo file I believe...to get the process started).