Messages in this thread | | | From | Christian Schoenebeck <> | Subject | Re: 9p caching with cache=loose and cache=fscache | Date | Wed, 29 Mar 2023 13:19:08 +0200 |
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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!
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