Re: 9p caching with cache=loose and cache=fscache

[Christian Schoenebeck <linux_oss@crudebyte.com>]

On Wednesday, March 29, 2023 12:08:26 AM CEST Dominique Martinet wrote:
> Luis Chamberlain wrote on Tue, Mar 28, 2023 at 10:41:02AM -0700:
> > >   "To speedup things you can also consider to use e.g. cache=loose instead.
> > 
> > My experience is that cache=loose is totally useless.
> 
> If the fs you mount isn't accessed by the host while the VM is up, and
> isn't shared with another guest (e.g. "exclusive share"), you'll get
> what you expect.
> 
> I have no idea what people use qemu's virtfs for but this is apparently
> common enough that it was recommended before without anyone complaining
> since that started being recommended in 2011[1] until now?
> 
> [1] https://wiki.qemu.org/index.php?title=Documentation/9psetup&diff=2178&oldid=2177
> 
> (now I'm not arguing it should be recommended, my stance as a 9p
> maintainer is that the default should be used unless you know what
> you're doing, so the new code should just remove the 'cache=none'
> altogether as that's the default.
> With the new cache models Eric is preparing comes, we'll get a new safe
> default that will likely be better than cache=none, there is no reason
> to explicitly recommend the historic safe model as the default has
> always been on the safe side and we have no plan of changing that.)

It's not that I receive a lot of feedback for what people use 9p for, nor am I
QEMU's 9p maintainer for a long time, but so far contributors cared more about
performance and other issues than propagating changes host -> guest without
reboot/remount/drop_caches. At least they did not care enough to work on
patches.

Personally I also use cache=loose and only need to push changes host->guest
once in a while.

> > >    That will deploy a filesystem cache on guest side and reduces the amount of
> > >    9p requests to hosts. As a consequence however guest might not see file
> > >    changes performed on host side *at* *all*
> > 
> > I think that makes it pretty useless, aren't most setups on the guest read-only?
> > 
> > It is not about "may not see", just won't. For example I modified the
> > Makefile and compiled a full kernel and even with those series of
> > changes, the guest *minutes later* never saw any updates.
> 
> read-only on the guest has nothing to do with it, nor has time.
> 
> If the directory is never accessed on the guest before the kernel has
> been built, you'll be able to make install on the guest -- once, even if
> the build was done after the VM booted and fs mounted.
> 
> After it's been read once, it'll stay in cache until memory pressure (or
> an admin action like umount/mount or sysctl vm.drop_caches=3) clears it.
> 
> I believe that's why it appeared to work until you noticed the issue and
> had to change the mount option -- I'd expect in most case you'll run
> make install once and reboot/kexec into the new kernel.
> 
> It's not safe for your usecase and cache=none definitely sounds better
> to me, but people should use defaults make their own informed decision.

It appears to me that read-only seems not to be the average use case for 9p,
at least from the command lines I received. It is often used in combination
with overlayfs though.

I (think) the reason why cache=loose was recommended as default option on the
QEMU wiki page ages ago, was because of its really poor performance at that
point. I would personally not go that far and discourage people from using
cache=loose in general, as long as they get informed about the consequences.
You still get a great deal of performance boost, the rest is for each
individual to decide.

Considering that Eric already has patches for revalidating the cache in the
works, I think the changes I made on the other QEMU wiki page are appropriate,
including the word "might" as it's soon only a matter of kernel version.

> >>   In the above example the folder /home/guest/9p_setup/ shared of the
> >>   host is shared with the folder /tmp/shared on the guest. We use no
> >>   cache because current caching mechanisms need more work and the
> >>   results are not what you would expect."
> >
> > I got a wiki account now and I was the one who had clarified this.
> 
> Thanks for helping making this clearer.

Yep, and thanks for making a wiki account and improving the content there
directly. Always appreciated!