[Libguestfs] [PATCH virt-v2v] v2v: Allow temporary directory to be set on a global basis.

Thu Apr 9 19:55:12 UTC 2020

On Tue, Apr 07, 2020 at 04:31:57PM +0200, Pino Toscano wrote:
> On Tuesday, 7 April 2020 14:59:11 CEST Tomáš Golembiovský wrote:
> > On Tue, Apr 07, 2020 at 02:33:20PM +0200, Pino Toscano wrote:
> > > On Tuesday, 7 April 2020 14:18:47 CEST Richard W.M. Jones wrote:
> > > > On Tue, Apr 07, 2020 at 01:25:02PM +0200, Pino Toscano wrote:
> > > > > The important thing is still that that you need to have space for the
> > > > > temporary files somewhere: be it /var/tmp, /mnt/scratch, whatever.
> > > > > Because of this, and the fact that usually containers are created
> > > > > fresh, the cache of the supermin appliance starts to make little sense,
> > > > > and then a very simple solution is to point libguestfs to that extra
> > > > > space:
> > > > > 
> > > > >   $ dir=$(mktemp --tmpdir -d /path/to/big/temporary/space/libguestfs.XXXXXX)
> > > > >   $ export LIBGUESTGS_CACHEDIR=$dir
> > > > >   $ export TMPDIR=$dir  # optionally
> > > > >   $ libguestfs-test-tool
> > > > >   [etc]
> > > > >   $ rm -rf $dir
> > > > > 
> > > > > Easy to use, already doable, solves all the issues.
> > > > 
> > > > So AIUI there are a few problems with this (although I'm still
> > > > investigating and trying to make a local reproducer):
> > > > 
> > > >  - The external large space may be on NFS, with the usual problems
> > > >    there like root_squash, no SELinux labels, slowness.  This means
> > > >    it's not suitable for the appliance, but might nevertheless be
> > > >    suitable for overlay files.
> > > 
> > > If that is the only big storage space attached to a container, I do
> > > not see any alternative than use it, with all the caveats associated.
> > 
> > I have to aggree with this. You cannot avoid situations where the
> > appliance is on a network drive. If there are any inherent risks the
> > best you can do is let user know about those (documentation?).
> > 
> > > 
> > > Also, if we take as possible scenario the situation where /var/tmp is
> > > not that big, then we need to consider that may not be big enough to
> > > even store the cached supermin appliance (which is more than
> > > 300/350MB).
> > > 
> > > >  - The external large space may be shared with other containers, and
> > > >    I'm not convinced that our locking in supermin will be safe if
> > > >    multiple parallel instances start up at the same time.  We
> > > >    certainly never tested it, and don't currently advise it.
> > > 
> > > That's why my suggestion above creates a specific temporary directory
> > > for each container: even with a shared /var/tmp, there will not be any
> > > cache stepping up on each other toes. This is something that this
> > > separate cachedir for virt-v2v does not solve at all.
> > 
> > Currently we don't share the temporary volume between instances in
> > Kubevirt, but the assumption that this can happen is valid and should be
> > considered.
> > 
> > > 
> > > > > This whole problem started from a QE report on leftover files after
> > > > > failed migrations: bz#1820282.
> > > > 
> > > > (I should note also there are two bugs which I personally think we can
> > > > solve with the same fix, but they are completely different bugs.)
> > > 
> > > I still do not understand how these changes have anything to do with
> > > bug 1814611, which in an offline discussion we found out that has
> > > mostly two causes:
> > > - the way the NFS storage is mounted over the /var/tmp in the
> > >   container, so what you create as root is not really with the UID/GID
> > >   expected
> > > - the fixed appliance in the container was not actually used, and thus
> > >   a rebuilt of the the supermin appliance was attempted, failing due
> > >   to the first issue
> > 
> > I am still not convinced this is the case. Based on the logs I shared in
> > private email I still believe that the fixed appliance was used
> > properly. You assumed that the appliance is not used because the cache
> > directory is being created, but as I also pointed out the cache
> > directory created in all situations because qemu temporary files are
> > stored there [1][2].
> 
> This may be the case, i.e. something else before even the supermin
> appliance check that triggers the creation of the cachedir.
> 
> In the end though, libguestfs prefers a supermin appliance before a
> fixed appliance; the whole logic is here:
> https://github.com/libguestfs/libguestfs/blob/c2c11382bbeb4500f3388a31ffd08cfc18b0de40/lib/appliance.c
> In particular, see the locate_or_build_appliance function and the
> comment before it, and contains_fixed_appliance &
> contains_supermin_appliance helper functions.

To avoid this, the fixed appliance was created in a separate directory,
then the content of /usr/lib64/guestfs was removed and finally the fixed
appliance was moved there.

> 
> If you have a setup like:
> 
> /usr/lib64/guestfs
> ├── initrd
> ├── kernel
> ├── README.fixed
> ├── root
> └── supermin.d
>     ├── base.tar.gz
>     └── packages
> 
> with LIBGUESTFS_PATH=/usr/lib64/guestfs, then the logic fill find first
> the two files under supermin.d as supermin appliance, and not even
> consider the fixed appliance there.
> 
> > 
> > > 
> > > Can you please explain me exactly how switching the location of
> > > temporary files (that were not mentioned in the bug at all) will fix
> > > this situation?
> > 
> > For the particular BZ 1814611, if we keep the fixed appliance we can
> > move the cache directory to something else than /var/tmp (just for the
> > qemu files).
> 
> Or simply move the location of the fixed appliance, and set
> LIBGUESTFS_PATH to it. I would do it regardless to avoid the situation
> described above, i.e. that the fixed appliance is shadowed by the
> supermin appliance because of the same location of both.

Yup, I have already heeded this advice and posted a patch which keeps
the appliance in /usr/lib64/guestfs.fixed and sets LIBGUESTFS_PATH.

    Tomas

> 
> > > > > What this report doesn't say, however,
> > > > > is that beside the mentioned files that virt-v2v creates, there are
> > > > > also leftover files that libguestfs itself creates. These files are
> > > > > usually downloaded from the guest for the inspection, and generally not
> > > > > that big compared to e.g. the overlays that virt-v2v creates.
> > > > > Nonetheless, an abrupt exit of virt-v2v will leave those in place, and
> > > > > they will still slowly fill up the space on /var/tmp (or whatever is
> > > > > the location of $LIBGUESTFS_CACHEDIR).
> > > > 
> > > > I guess that small files being left around aren't really a problem.
> > > > The problem they have is large files being left around, and I can
> > > > understand why that would be an issue and not the small files.
> > > 
> > > Nobody is saying that the leftover files are not a problem. I'm saying
> > > that also the small files are a sort of problem -- sure, less critical,
> > > however still there and ready to show up any time, especially if the
> > > concern is the space of /var/tmp.
> > 
> > Isn't that the reason why things like tmpreaper exist?
> 
> Then we can apply that also for the virt-v2v files in general, running
> it for any content older than 2/3 days. Even better, with the proposed
> consolidation of temporary directories [1], tmpreaper can be pointed at
> those directories only, not touching anything else.
> 
> [1] https://www.redhat.com/archives/libguestfs/2020-April/msg00050.html
> 
> -- 
> Pino Toscano

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-- 
Tomáš Golembiovský <tgolembi at redhat.com>