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

[Pino Toscano]

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)

-- 
Pino Toscano
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