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

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/c2c11382bbeb4500f3388a31ffd... In particular, see the locate_or_build_appliance function and the comment before it, and contains_fixed_appliance & contains_supermin_appliance helper functions.

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.

> > > > 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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