+CC: Jeff for experience on this caching issue with NFS ... On Tuesday, March 28, 2023 5:51:51 PM CEST Eric Van Hensbergen wrote: > As I work through the documentation rework and to some extent the > testing matrix -- I am reconsidering some choices and wanted to open > up the discussion here. > > TLDR; I'm thinking of reworking the cache options before the merge > window to keep things simple while setting up for some of the future > options. Yeah, revising the 9p cache options highly makes sense! So what is the plan on this now? I saw you sent a new patch with the "old" options today? So is this one here deferred? > While we have a bunch of new options, in practice I expect users to > probably consolidate around three models: no caching, tight caches, > and expiring caches with fscache being an orthogonal add-on to the > last two. Actually as of today I don't know why somebody would want to use fscache instead of loose. Does it actually make sense to keep fscache and if yes why? > The ultimate goal is to simplify the options based on expected use models: > > - cache=[ none, file, all ] (none is currently default) dir? > - write_policy = [ *writethrough, writeback ] (writethrough would be default) > - cache_validate = [ never, *open, x (seconds) ] (cache_validate > would default to open) Jeff came up with the point that NFS uses a slicing time window for NFS. So the question is, would it make sense to add an option x seconds that might be dropped soon anyway? > - fscache > > So, mapping of existing (deprecated) legacy modes: > - none (obvious) write_policy=writethrough > - *readahead -> cache=file cache_validate_open write_policy=writethrough > - mmap -> cache=file cache_validate=open write_policy=writeback Mmm, why is that "file"? To me "file" sounds like any access to files is cached, whereas cache=mmap just uses the cache if mmap() was called, not for any other file access. > - loose -> cache=all cache_validate=never write_policy=writeback > - fscache -> cache=all cache_validate=never write_policy=writeback & > fscache enabled > > Some things I'm less certain of: cache_validation is probably an > imperfect term as is using 'open' as one of the options, in this case > I'm envisioning 'open' to mean open-to-close coherency for file > caching (cache is only validated on open) and validation on lookup for > dir-cache coherency (using qid.version). Specifying a number here > expires existing caches and requires validation after a certain number > of seconds (is that the right granularity)? Personally I would then really call it open-to-close or opentoclose and waste some more characters in favour of clarity. > So, I think this is more clear from a documentation standpoint, but > unfortuantely I haven't reduced the test matrix much - in fact I've > probably made it worse. I expect the common cases to basically be: > - cache=none > - new default? (cache=all, write_policy=writeback, cache_validate=open) > - fscache w/(cache=all, write_policy=writeback, cache_validate=5) > > Which would give us 3 configurations to test against versus 25 > (assuming testing for one time value for cache-validate=x). Important > to remember that this is just cache mode tests, the other mount > options act as multipliers. > > Thoughts? Alternatives? > > -eric > > On Mon, Mar 27, 2023 at 10:38 AM Christian Schoenebeck > <linux_oss@xxxxxxxxxxxxx> wrote: > > > > On Monday, March 27, 2023 5:05:52 AM CEST Eric Van Hensbergen wrote: > > > Need to update the documentation for new mount flags > > > and cache modes. > > > > > > Signed-off-by: Eric Van Hensbergen <ericvh@xxxxxxxxxx> > > > --- > > > Documentation/filesystems/9p.rst | 29 ++++++++++++++++------------- > > > 1 file changed, 16 insertions(+), 13 deletions(-) > > > > > > diff --git a/Documentation/filesystems/9p.rst b/Documentation/filesystems/9p.rst > > > index 0e800b8f73cc..6d257854a02a 100644 > > > --- a/Documentation/filesystems/9p.rst > > > +++ b/Documentation/filesystems/9p.rst > > > @@ -78,19 +78,18 @@ Options > > > offering several exported file systems. > > > > > > cache=mode specifies a caching policy. By default, no caches are used. > > > - > > > - none > > > - default no cache policy, metadata and data > > > - alike are synchronous. > > > - loose > > > - no attempts are made at consistency, > > > - intended for exclusive, read-only mounts > > > - fscache > > > - use FS-Cache for a persistent, read-only > > > - cache backend. > > > - mmap > > > - minimal cache that is only used for read-write > > > - mmap. Northing else is cached, like cache=none > > > + Modes are progressive and inclusive. For example, specifying fscache > > > + will use loose caches, writeback, and readahead. Due to their > > > + inclusive nature, only one cache mode can be specified per mount. > > > > I would highly recommend to rather specify below for each option "this option > > implies writeback, readahead ..." etc., as it is not obvious otherwise which > > option would exactly imply what. It is worth those extra few lines IMO to > > avoid confusion. > > > > > + > > > + ========= ============================================= > > > + none no cache of file or metadata > > > + readahead readahead caching of files > > > + writeback delayed writeback of files > > > + mmap support mmap operations read/write with cache > > > + loose meta-data and file cache with no coherency > > > + fscache use FS-Cache for a persistent cache backend > > > + ========= ============================================= > > > > > > debug=n specifies debug level. The debug level is a bitmask. > > > > > > @@ -137,6 +136,10 @@ Options > > > This can be used to share devices/named pipes/sockets between > > > hosts. This functionality will be expanded in later versions. > > > > > > + directio bypass page cache on all read/write operations > > > + > > > + ignoreqv ignore qid.version==0 as a marker to ignore cache > > > + > > > noxattr do not offer xattr functions on this mount. > > > > > > access there are four access modes. > > > > > > > > > > > >
