linux-kernel - Re: 9p caching with cache=loose and cache=fscache

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Date:   Tue, 28 Mar 2023 10:41:02 -0700
From:   Luis Chamberlain <mcgrof@...nel.org>
To:     Christian Schoenebeck <linux_oss@...debyte.com>
Cc:     Dominique Martinet <asmadeus@...ewreck.org>,
        Eric Van Hensbergen <ericvh@...il.com>,
        Josef Bacik <josef@...icpanda.com>,
        Jeff Layton <jlayton@...nel.org>, lucho@...kov.net,
        v9fs-developer@...ts.sourceforge.net, linux-kernel@...r.kernel.org,
        Amir Goldstein <amir73il@...il.com>,
        Pankaj Raghav <p.raghav@...sung.com>
Subject: Re: 9p caching with cache=loose and cache=fscache

On Tue, Mar 28, 2023 at 01:53:49PM +0200, Christian Schoenebeck wrote:
> Hi Luis,
> 
> not sure which QEMU wiki page you are referring to. AFAIK we currently have 3
> QEMU wiki pages concerning 9p:
> 
> 1. 9p documentation for users:
> https://wiki.qemu.org/Documentation/9psetup

It was this one. I hadn't looked at the other ones.

> 2. 9p documentation for developers only:
> https://wiki.qemu.org/Documentation/9p
> 
> 3. How to setup an entire guest on top of a 9p root filesystem:
> https://wiki.qemu.org/Documentation/9p_root_fs
> 
> Only the latter wiki page mentions cache=loose at all:
> 
>   "To speedup things you can also consider to use e.g. cache=loose instead. 
>    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 
>    immediately see file changes performed on host side. So choose wisely upon
>    intended use case scenario. You can change between cache=mmap or e.g.
>    cache=loose at any time."
> 
> Which I now changed to:
> 
>   "To speedup things you can also consider to use e.g. cache=loose instead.

My experience is that cache=loose is totally useless.

>    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.

> (as Linux kernel's 9p client 
>    currently does not revalidate for fs changes on host side at all, which is
>    planned to be changed on Linux kernel side soon though). So choose wisely
>    upon intended use case scenario. You can change between cache=mmap or e.g.
>    cache=loose at any time."
> 
> On the user page it was already clearly mentioned though:
> 
>   "Mount the shared folder on guest using
> 
>       mount -t 9p -o trans=virtio test_mount /tmp/shared/ -oversion=9p2000.L,posixacl,msize=104857600,cache=none
> 
>   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.

  Luis