Re: kern/53616: dom0 crashes when dumping domU images with vnd+fss

[Robert Elz <kre%munnari.OZ.AU@localhost>]

The following reply was made to PR kern/53616; it has been noted by GNATS.

From: Robert Elz <kre%munnari.OZ.AU@localhost>
To: =?UTF-8?Q?Fr=C3=A9d=C3=A9ric_Fauberteau?= <triaxx%NetBSD.org@localhost>
Cc: gnats-bugs%netbsd.org@localhost
Subject: Re: kern/53616: dom0 crashes when dumping domU images with vnd+fss
Date: Wed, 19 Sep 2018 13:37:18 +0700

     Date:        Wed, 19 Sep 2018 07:03:28 +0200
     From:        =3D?UTF-8?Q?Fr=3DC3=3DA9d=3DC3=3DA9ric_Fauberteau?=3D <t=
 riaxx=40NetBSD.org>
     Message-ID:  <286f177771acf1fe1766d1a29ef7bd60=40NetBSD.org>
 
   =7C Yes they run. But my idea was to not modify the mounted fs and to d=
 ump a=20
   =7C snapshot.
 
 As Manuel indicated, the issue is that making the snapshot itself modifie=
 s
 the filesystem - I have no idea how linux might cope otherwise, but the
 filesystem (and the OS that is actually using it) needs to act differentl=
 y when
 there'=5Bs a snapshot, as it needs to preserve the previous data, rather =
 that
 simply overwriting it.   Hence, it has to know that the snapshot has been=
 
 made, and making one while a system is running cannot possibly work.
 
 It is possible that snapshotting an LVM might work, though I have no idea=
 
 if we support that  - thyat would basically modify the block map undernea=
 th
 the filesystem, and should be possible (if implemented...)   Bit not the
 filesystem itself.
 
   =7C I will test again by explicitly mounting my fs in read-only mode.
 
 If you mean mounting the fs via a vnd read only, no, that won't help.
 It might let  you access the filesystem without causing the rest of the
 system to fail, but that system would not remain consistent, and your
 backup would likely be nonsense.   If you meant moounting the snapshot
 read only, I don't think there's a choice - whether you give the read
 only option or not, that's what happens anyway (and that would not
 help.)
 
   =7C Maybe I misunderstand and one of the commands I used does syscalls =
 that=20
   =7C modify fs.
 
 Yes, the ones that make the snapshot.
 
 Manuel's idea of snapshotting the filesystem that contains the images of =
 the
 files that are the backing store (if you are using file based storage) is=
 =20
 probably the best that you can currently expect to worl.   If your system=
 s are
 mapping partitions though, that won't work.
 
 kre