Hi David-
On Dec 5, 2007, at 8:22 PM, David Howells wrote:
Chuck Lever <[email protected]> wrote:
I don't see how persistent local caching means we can no longer
ignore (a)
and (b) above. Can you amplify this a bit?
How about I put it like this. There are two principal problems to
be dealt
with:
(1) Reconnection.
Imagine that the administrator requests a mount that uses part
of a cache.
The client machine is at some time later rebooted and the
administrator
requests the same mount again.
Since the cache is meant to be persistent, the administrator
is at liberty
to expect that the second mount immediately begins to use the
data that
the first mount left in the cache.
For this to occur, the second mount has to be able to
determine which part
of the cache the first mount was using and request to use the
same piece
of cache.
To aid with this, FS-Cache has the concept of a 'key'. Each
object in the
cache is addressed by a unique key. NFS currently builds a
key to the
cache object for a file from: "NFS", the server IP address,
port and NFS
version and the file handle for that file.
Why not use the fsid as well? The NFS client already uses the fsid
to detect when it is crossing a server-side mount point. Fsids are
supposed to be stable over server reboots (although sometimes they
aren't, it could be made a condition of supporting FS-cache on clients).
I also note the inclusion of server IP address in the key. For multi-
homed servers, you have the same unavoidable cache aliasing issues if
the client mounts the same server and export via different server
network interfaces.
(2) Cache coherency.
Imagine that the administrator requests a mount that uses part
of a
cache. The administrator then makes a second mount that
overlaps the
first, maybe because it's a different part of the same server
export or
maybe it uses the same part, but with different parameters.
Imagine further that a particular server file is accessible
through both
mountpoints. This means that the kernel, and therefore the
user, has two
views of the one file.
If the kernel maintains these two views of the files as
totally separate
copies, then coherency is mostly not a kernel problem, it's an
application
problem - as it is now.
However, if these two views are shared at any level - such as
if they
share an FS-Cache cache object - then coherency can be a problem.
Is it a problem because, if there are multiple copies of the same
remote file in its cache, then FS-cache doesn't know, upon
reconnection, which item to match against a particular remote file?
I think that's actually going to be a fairly typical situation --
you'll have conditions where some cache items will become orphaned,
for example, so you're going to have to deal with that ambiguity as a
part of normal operation.
For example, if the FS-caching client is disconnected or powered off
when a remote rename occurs that replaces a file it has cached, the
client will have an orphaned item left over. Maybe this use case is
only a garbage collection problem.
The two simplest solutions to the coherency problem are (a) to
enforce
sharing at all levels (superblocks, inodes, cache objects),
(b) to enforce
non-sharing. In-between states are possible, but are much
trickier and
more complex.
Note that cache coherency management can't be entirely
avoided: upon
reconnection a cache object has to be checked against the
server to see
whether it's still valid.
How do you propose to do that?
First, clearly, FS-cache has to know that it's the same object, so
fsid and filehandle have to be the same (you refer to that as the
"reconnection problem", but it may generally be a "cache aliasing
problem").
I assume FS-cache has a record of the state of the remote file when
it was last connected -- mtime, ctime, size, change attribute (I'll
refer to this as the "reconciliation problem")? Does it, for
instance, checksum both the cache item and the remote file to detect
data differences?
You have the same problem here as we have with file system search
tools such as Beagle. Reconciling file contents after a reconnection
event may be too expensive to consider for NFS, especially if a file
is terabytes in size.
Note that both these problems only really exist because the cache is
persistent between mounts. If it were volatile between mounts,
then (1) would not exist, and (2) can be ignored as it is now.
Do you allow administrators to select whether the FS-cache is
persistent? Or is it always unconditionally persistent?
An adequate first pass at FS-cache can be done without guaranteeing
persistence. There are a host of other issues that need exposure --
steady-state performance; cache garbage collection and reclamation;
cache item aliasing; whether all files on a mount point should be
cached on disk, or some in memory and some on disk; and so on -- that
can be examined without even beginning to worry about reboot recovery.
And what would it harm if FS-cache decides that certain items in its
cache have become ambiguous or otherwise unusable after a
reconnection event, thus it reclaims them instead of re-using them?
There are three obvious ways of dealing with the problems (ignoring
the fact
that all cases have on-reconnection coherency to deal with whatever):
(a) No sharing at all.
Cache coherency is what it is now with NFS, but reconnection
must be
managed. A key must be generated to each mount to distinguish
that mount
from an overlapping mount that might contain the same files.
These keys must be unique (and uniqueness must be enforced)
unless two
superblocks are guaranteed disjoint (eg: on different
servers), or are
guaranteed to share anyway (eg: exact same parameter sets and
nosharecache
not specified).
(b) Fully shared.
Cache coherency is a non-issue. Reconnection is a non-issue.
Any
particular server inode is guaranteed to be represented by a
single inode
on the client, both in the superblock and the pagecache, and
by a single
FS-Cache cache object.
The downside of this is that sharing must take priority over
different
connection parameters. R/O vs R/W can be dealt relatively
easily as I
believe it's a local phenomenon, and is dealt with before the
filesystem
is consulted. There are patches to do this.
(c) Implicit full sharing between cached mountpoints; uncached
mountpoints
need not be shared.
Cached mountpoints have the properties of (b), uncached
mountpoints are
left to themselves.
Note that redundant disk usage is undesirable, but unlikely to
cause a real
problem, such as an oops. Non-unique keys, on the other hand, are
a problem.
Having non-shared local inodes sharing cache objects causes even
more problems,
and I don't want to go there.
Nothing you say in the rest of your proposal convinces me that having
multiple caches for the same export is really more than a
theoretical issue.
Okay. So how do you do reconnection?
The simplest way from what I see is to require that the
administrator specify
everything, but this is probably not what you want if you're
distributing NFS
mounts by NIS, say.
Automatic configuration is preferred. For example, NFS with Kerberos
has an administrative scaling problem because some local
administration (creating a keytab and registering the client with
KDC) is required for every client that joins a realm.
The next simplest way is to bind all the differentiation parameters
(see
nfs_compare_mount_options()) into a key and use that, plus a
uniquifier from
the administrator if NFS_MOUNT_UNSHARED is set.
It gives us the proper legacy behavior, but as soon as the
administrator changes a mount option, all previously cached items for
that mount point become orphans.
As useful as the feature is, one can also argue that mounting the
same export
multiple times is infrequent in most normal use cases.
Practically speaking,
why do we really need to worry about it?
Because it's possible. Because it has to be considered. Because,
as you said,
people do it. Because if I don't deal with it, the kernel will
oops when NFS
asks FS-Cache to do something it doesn't support.
I can't just say: "Well, it'll oops if you configure your NFS
shares like that,
so don't. It's not worth me implementing round it.".
What causes that instability? Why can't you insulate against the
instability but allow cache incoherence and aliased cache items?
Local file systems are fraught with cases where they protect their
internal metadata aggressively at the cost of not keeping the disk up
to date with the memory version of the file system.
Similar compromises might benefit FS-cache. In other words, FS-cache
for NFS file systems may be less functional than for, say, AFS, to
allow the cache to operate reliably.
The real problem here is that the NFS protocol itself does not
support strong
cache coherence. I don't see why the Linux kernel must fix that
problem.
So you're arguing there shouldn't be local caching for NFS? Or
that there
shouldn't be persistent local caching for NFS?
I'm arguing that cache coherence isn't supported by the NFS protocol,
so how can FS-cache *require* a facility to support persistent local
caching that the protocol doesn't have in the first place?
NFS client implementations do the best they can; there are always
scenarios where coherence issues cause behavior no-one expects.
Usually NFS clients handle ambiguous cases by invalidating their
caches. Invalidating is cheap for in-memory caches. Frequent
invalidation is going to be expensive for FS-cache, since it requires
some disk I/O (and perhaps even file truncation). One reason why
chunk caching is better than whole-file caching is that it bounds the
time and effort to recycle a cache item.
AFS assigns universally unique identities to servers, volumes, and
files. NFS doesn't guarantee unique identities to servers or
exports, and file handles are supposed to be unique only on a given
server [*]. And unfortunately file handles can be re-used by the
server without any indication to the client that the file handle it
has cached is no longer the same file (see the "out_fileid" label in
fs/nfs/inode.c:nfs_update_inode). AFS provides client-visible
generation IDs in its inode numbers for this case.
Thus NFS itself does not provide any good way to help you sort FS-
cache cache items outside of a single export. A proper FS-cache
implementation thus cannot depend on server/export identity to
guarantee the singularity of cache items.
So FS-cache will have a hard time guaranteeing that there is only one
item in its cache that maps to a given NFS server file. It may also
be difficult to guarantee that multiple NFS server files do not map
onto the same local cache item (file handle re-use).
This suggests to me that the cache aliasing problem is unsolvable for
NFS, so you should find a way to make FS-cache work in a world where
cache aliasing is a fact of life.
Lastly, there's already a mount option that allows admins to
control whether
the page and attribute caches are shared -- "sharecache". Is
this mount
option not adequate for persistent caching?
Adequate in what way? It doesn't currently automatically guarantee
sharing of
overlapping superblocks. It merely disables nonsharecache which
explicitly
disables cache sharing.
The current problem with "sharecache" is that the mount options on
subsequent mounts of the same export are silently ignored. You are
proposing the same behavior for FS-cache-managed mount points, which
means we're spreading bad UI behavior further.
At least there should be a warning that explains why a file system
that was mounted with "rw,noac,tcp" is behaving like it's "ro,ac,udp".
Ideally, if we must have cache sharing, the behavior should be: if
the mount options, the server, and the fsid are the same, then the
cache should be shared. If any of that tuple are different, then a
unique cache is used for that mount point (within the limits of being
able to determine the unique identity of a server and export).
--
Chuck Lever
chuck[dot]lever[at]oracle[dot]com
[*] Section 4 of RFC 3530 states:
The filehandle in the NFS protocol is a per server unique identifier
for a filesystem object. The contents of the filehandle are opaque
to the client. Therefore, the server is responsible for translating
the filehandle to an internal representation of the filesystem object.
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