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

[Richard W.M. Jones]

On Thu, Apr 02, 2020 at 03:21:14PM +0200, Pino Toscano wrote:
> On Thursday, 2 April 2020 14:49:18 CEST Richard W.M. Jones wrote:
> > Previously we placed large files in g#get_cachedir () (usually
> > /var/tmp).  However the problem is this ties the libguestfs appliance
> > and the virt-v2v overlay files to the same location.
> > 
> > When virt-v2v is run in a container, or any other situation where
> > local storage is limited, it's helpful to be able to put the overlay
> > files on an externally mounted PVC, which might be using NFS and
> > shared between containers.  But putting the libguestfs appliance on
> > NFS in a shared location is certainly not recommended.
> > 
> > This allows the two locations to be set separately:
> > 
> >   VIRT_V2V_TMPDIR - location of large temporary files, can use NFS
> >                     and may be shared
> > 
> >   LIBGUESTFS_CACHEDIR - location of libguestfs appliance
> > 
> > Another motivation for this patch is to allow more reliable cleanup of
> > temporary files by an external process, as described in the updated
> > documentation.
> > ---
> 
> I do not understand the motivation behind this, which adds yet another
> location with temporary files in addition to:
> - LIBGUESTFS_TMPDIR - $TMPDIR by default (which itself is /tmp by
>   default)
> - LIBGUESTFS_CACHEDIR - /var/tmp by default (with a .guestfs-$UID
>   subdirectory for the appliance)
> 
> Before this patch, almost all the temporary files are stored directly
> or in subdirectories of $TMPDIR, except big files such as overlays and
> OVA extracted content that are in CACHEDIR. With the proposed changes,
> _all_ the temporary files will be in CACHEDIR, so there are the
> following problems:
> - this directory will be cluttered with a lot more files than before
> - if it is shared, then other places where it is mounted will see the
>   same files
> - if it is shared, then creating temporary files will possibly mean
>   doing network I/O
> - if virt-v2v exits uncleantly, there will be a lot more files to
>   cleanup than now
> - even without being shared, /var/tmp is persistent unlike /tmp (which
>   can be tmpfs-backed on some distros/setups), meaning old temporary
>   files will linger way more

How about if we confine the change to just large files (ie. ones
which are currently placed in cachedir)?

> That said, it is not clear to me why /var/tmp should be shared among
> containers.

I also don't think /var/tmp should be shared, nor use NFS.

However the way that virt-v2v works at the moment means you cannot put
the large files (especially v2vovl*.qcow2) in a different place from
the libguestfs appliance.  This means that if you have only "some"
space in /var/tmp -- enough for the appliance, but not enough for the
potentially much larger space required by v2vovl*.qcow2 with multiple
copies of virt-v2v running -- then you cannot separate the overlays to
another directory.  This isn't just a problem for containers.

Another issue here is that with kubenetes it's not up to the tenant to
choose what backing store is used for volume mounts.  So if the
administrator of the cluster set it up with NFS, then that's what you
have to use.  (However using NFS for /var/tmp is still wrong).

That's the problem we're trying to solve.

> > diff --git a/docs/virt-v2v.pod b/docs/virt-v2v.pod
> > index ed95fdc8e..dbfd10cad 100644
> > --- a/docs/virt-v2v.pod
> > +++ b/docs/virt-v2v.pod
> > @@ -1206,8 +1206,9 @@ possible.
> >  
> >  =head3 Disk space
> >  
> > -Virt-v2v places potentially large temporary files in C<$TMPDIR> (which
> > -is F</var/tmp> if you don't set it).  Using tmpfs is a bad idea.
> 
> Regardless of this patch, this bit is not correct:
> - libguestfs does not places large files in $TMPDIR (but in CACHEDIR)
> - $TMPDIR is not /var/tmp by default, but /tmp

Yes this needs fixing separately.  I forgot $TMPDIR is not
related to /var/tmp.

Rich.

-- 
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