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

[Daniel P. Berrangé]

On Fri, Apr 03, 2020 at 12:53:14PM +0100, Richard W.M. Jones wrote:
> On Thu, Apr 02, 2020 at 05:51:32PM +0200, Pino Toscano wrote:
> > On Thursday, 2 April 2020 17:30:39 CEST Richard W.M. Jones wrote:
> > > 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)?
> > 
> > This is already the case, isn't it?
> > 
> > > 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.
> > 
> > /var/tmp is a temporary directory, hence it ought to have enough space
> > for big temporary files. This is nothing special for libguestfs or
> > virt-v2v.
> > 
> > I do not see what makes v2vovl*.qcow2 files so special in this regard,
> > requiring them to be handled specially than other big temporary files
> > already stored by libguestfs/virt-v2v. Examples:
> > - the appliance
> > - the temporary directory for qemu when using the libvirt backend
> > - the extracted content of an OVA, in case we cannot read it directly
> > - the disks when exporting to glance (-o glance)
> 
> "Because containers" is the only answer I can give to this.
> Containers have weird constraints, in that /var/tmp may be
> big, but not big enough for the overlay files.
> 
> I still think the principle of being able to put the
> libguestfs appliance and the other big temporary files
> in different places is not a bad one.

Yeah, it makes conceptual sense to me to be able to control where
v2v puts its temporary disks explicitly, and not have this location
tied to libguestfs' temporary location since they are separate
components.

Regards,
Daniel
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