CITI Experience with Directory Delegations
From Linux NFS
NOTE: this is a rough work-in-progress; please send criticism to richterd at (nospam) citi.umich.edu thank you.
[2006-8-2: I've added some rough, preliminary numbers of opcounts from doing compiles with/without directory delegations]
Directory Delegations Background
NFSv4.1 introduces read-only directory delegations, a protocol addition intended to enable clients to perform more-aggressive caching. More specifically, the goal is essentially to allow clients to avoid excess GETATTR, ACCESS, and LOOKUP calls to the server by increasing the reliability with which clients are able to do directory entry caching (READDIR), name caching (LOOKUP), and directory metadata caching (ACCESS and GETATTR).
The following quoted subsections are from Section 11 of the NFSv4.1 minor version draft:
NFSv4 client caching behavior
"Directory caching for the NFS version 4 protocol is similar to previous versions. Clients typically cache directory information for a duration determined by the client. At the end of a predefined timeout, the client will query the server to see if the directory has been updated. By caching attributes, clients reduce the number of GETATTR calls made to the server to validate attributes. Furthermore, frequently accessed files and directories, such as the current working directory, have their attributes cached on the client so that some NFS operations can be performed without having to make an RPC call. By caching name and inode information about most recently looked up entries in DNLC (Directory Name Lookup Cache), clients do not need to send LOOKUP calls to the server every time these files are accessed."
NFSv4.1 delegations extensions
"[The NFSv4] caching approach works reasonably well at reducing network traffic in many environments. However, it does not address environments where there are numerous queries for files that do not exist. In these cases of "misses", the client must make RPC calls to the server in order to provide reasonable application semantics and promptly detect the creation of new directory entries. Examples of high miss activity are compilation in software development environments. The current behavior of NFS limits its potential scalability and wide-area sharing effectiveness in these types of environments."
Furthermore, analysis of NFSv3 (whose client cache semantics NFSv4 mirrors) network traces by Brian Wickman at the University of Michigan (FIXME: need link to a copy of his prelim) show that a very surprising amount of NFS traffic are the periodic GETATTRs the clients send when a timeout triggers a cache revalidation.
At CITI, we are in the process of implementing directory delegations as described in Section 11 of the minor version draft, although we are not at this time implementing the notifications extension also described therein. The following are some specific aspects of the work.
Using Directory Delegations
While a client holds a delegation on a directory, it is assured that the directory will not be mutated without the delegation first being synchronously recalled. However, the notion that a directory delegation is "read-only" has a special case associated with it: a given client's mutating operation on a directory delegated to it will not trigger a recall of that client's delegation (obviously, all other delegations on that directory will have to be recalled before the mutating operation succeeds). Again, from Section 11 of the minor version draft:
"The delegation is read-only and the client may not make changes to the directory other than by performing NFSv4 operations that modify the directory or the associated file attributes so that the server has knowledge of these changes."
Note that a client's ability to mutate a directory without triggering a recall of its own delegation is not at all a write delegation.
if i understand you correctly, the server currently synchronously
revokes the delegation from each client. (synchronously, but
hopefully not serially.) you are proposing that the server skip
over the mutating client, which sounds fine. perhaps it should
automatically grant a fresh delegation along with its ack to the
client update.
(Re: first paragraph) Yes, before the directory-mutating operation succeeds, all outstanding delegations
will be recalled (in parallel) -- except for a delegation held by the directory-mutating client, if such
a delegation exists.
I don't quite understand automatically granting a fresh delegation to the mutating owner-client -- do you
mean renew the client's lease (not a Linux VFS lease; the NFS lease that determines if a client is
non-responsive)? Can you help me understand? The client's existing delegation should suffice and be
basically indistinguishable from a new one. thanks, -d
my further understanding is that at the time that the server is busily revoking
delegations, it does not know which is the mutating client, and that sessions
is the magic bullet that solves the problem. yes?
(Re: second paragraph) Yup. For the time-being, I have a hack that conflates a client's IP address with
that client's clientid -- when the server receives SETCLIENTID_CONFIRM, it logs the client's IP for later
use. When a delegation-breaking operation comes in, the requester's IP is used to find the clientid. This
is a hack for prototyping's sake, because in practice multiple clientids can be associated with a given IP.
With Sessions, we'll have a session-related ID that will properly map to the requester's clientid.
Delegations and the Linux VFS Lease Subsystem
Directory delegations are implemented on the server with extensions to the Linux VFS file lease subsystem. A lease is a type of lock that gives the lease-holder the chance to perform any necessary tasks (e.g., flushing data) when an operation that conflicts with the lease-type is about to occur -- the caller who is causing the lease to break will block until the lease-holder signals that it is finished cleaning-up (or the lease is forcefully broken after a certain timeout).
The existing lease subsystem only works on files, and leases are only broken when a file is opened for writing or is truncated. In order to implement directory delegations, we have added support for directory leases. These will break when a leased directory is mutated by any additions, deletions, renames, or when the directory's own metadata changes (e.g., chown(1)). Note that changes to existing files, e.g., will not break directory leases.
However, for the very near-term, only NFS protocol operations break directory leases. There are a couple operations that involve some tricky locking issues in the VFS which will be addressed. The difficulty is that, given that breaking a lease involves blocking the caller, one must ensure that no important locks -- like a directory inode's i_mutex -- are held while the calling kernel thread blocks.
UPDATE
At this point, we are testing general VFS-level directory lease-breaking -- i.e., both NFS and non-NFS operations will break leases. Our approach is described in the next section.
Leases are usually acquired via the fcntl(2) call, and a lease-holder usually receives a signal from the kernel when a lease is being broken; the lease-holder indicates that any cleanup is finished with another fcntl(2) call. NFS leases are all acquired and revoked in-kernel.
Recalling NFS Delegations vs. Breaking Linux VFS (Non-NFS) Leases
The NFS delegation leases and existing non-NFS leases differ in how they handle the case where a lease-holder is also the caller performing an operation that conflicts with the lease-type. (Here I will informally refer to leases used to implement delegations as "NFS leases" and all other leases as "non-NFS leases".)
With NFS leases, in order to support directory delegation semantics, the caller a) will not have its own directory lease broken, and b) it will not block until all other leases on that directory have been returned to the server, and so c) the operation fails with NFS4ERR_DELAY, which indicates that the client should retry the operation. Bruce mentions that we could, perhaps, stall the operation for something like 100ms just in case the delegation(s) is/are returned very quickly.
With non-NFS leases, the caller a) will have its lease revoked, and b) will not block while any other lease-holders return their leases, with the latter implying that c) the operation fails with -EAGAIN.
UPDATE
The approach we're currently taking to tackle the issues of integrating NFS delegations with Linux VFS leases (i.e., all directory mutating
operations, whether locally on the server or over NFS, will break directory leases/delegations on the server) goes something like this:
When breaking a lease where the call is coming over NFS:
1) During processing, whenever the directory's dentry becomes available (e.g., after a lookup), disable lease-granting for its inode and try
break_lease() with O_NONBLOCK. This will avoid blocking while locks are held, as well as avoid tying up server threads for (potentially)
long periods.
2) If there was not a lease, finish the operation, re-enable lease-granting on the inode, and we're done.
3) If there was a lease, break_lease() will send the break signal(s) and nfsd will also fail (re-enabling lease-granting on the inode first)
and the client gets NFS4ERR_DELAY (and should retry). The downside to this is that a pathological case could arise wherein we break a lease,
return NFS4ERR_DELAY, then the client retries the operation -- but another client has acquired a lease in the interim, and we could end up
with a cycle.
When breaking a lease where the call is server-local:
1) Again, whenever a directory's dentry becomes available, disable lease-granting for its inode.
2) If locks (e.g., an i_mutex) are not held, call break_lease() and, as per normal lease-semantics, block the breaker until leases are returned,
after which the breaker is unblocked and its operation succeeds.
3) If locks are held, call break_lease() with O_NONBLOCK; we assume the common-case to be that no lease is present. If break_lease() returns
-EWOULDBLOCK, drop the locks and call break_lease() and allow it to block. Once the caller unblocks, restart the operation by reacquiring
the locks and, e.g., redoing a lookup to make sure the file system object(s) still exist(s). Since lease-granting was disabled early-on,
the operation will succeed in one pass.
4) Regardless of whether 2) or 3) happened, at the end lease-granting is naturally re-enabled for the inode(s) in question.
Negative Caching
One opportunity offered by directory delegations is the chance to significantly extend the usefulness of negative dentry caching on the client. Currently, close-to-open consistency requires that, e.g., all OPENs are sent to the server (i.e., negative caching provides no benefit in that case). With directory delegations, one is assured that no new entries or removals have occurred while a delegation is in-effect; this implies that negative dentries in a delegated directory actually can be "trusted".
This could translate into a marked decrease in the number of unnecessary and repeated checks for non-existent files, e.g. when searching for an executable in PATH or a shared library in LD_LIBRARY_PATH. Knowing just when to acquire those delegations may be a matter to address in client-side policy.
Status
At the moment, working on coming up with reasonably representative tests that show the benefits of directory delegations (in terms of OP-counts); pynfs tests are also being written.
The client:
- The client currently requests a delegation just prior to issuing a READDIR on an undelegated directory, or when it has done "a few" parent directory revalidations and noticed that it hasn't changed during that span.
- As long as the client has such a delegation, it will generally refrain from issuing ACCESS, GETATTR, and READDIR calls on the directory (see below) ...
- .. in some cases, though, the client's cache(s) may be deliberately invalidated and require a refresh (e.g., a client creates a file in a directory delegated to it, which won't break its delegation; however, in order to see the file, the client must revalidate its pagecache and send a READDIR on the wire).
- TODO: need to test the client's behavior with negative dentries in delegated directories.
- TODO: get some opcounts!
The server:
- Differentiate between turning file/directory delegations on/off at runtime (done) and enabling/disabling the capability itself (not done; would prevent our client from ever asking for delegations in the first place, independent of its requesting policy).
- The following NFS operations currently break directory delegations: CREATE, LINK, REMOVE, RENAME, and OPEN(w/create). SETATTR on directories is pending.
- An NFS SETATTR breaks file delegations when the file size is changing. Breaking on metadata changes is pending.
- The corresponding VFS-level operations also break delegations and are being tested.
- TODO: get NFSv2/NFSv3 operations to break (file and directory) delegations at all of the right times, too.
- TODO: also -- policy, look at dir deleg/file deleg interactions, ..
Some preliminary numbers
We have some very rough numbers in terms of opcounts with vs. without directory (not file) delegations enabled. We used a very naive client policy of simply requesting delegations prior to a READDIR (note that make(1) periodically calls getdents(2) on its own). ACCESS, GETATTR, and LOOKUP are where the real savings are; the other opcounts are just included for context. Again, these numbers are rough.
Doing make(1) on cscope-15.5 (first with directory delegations, then without):
READ: 124 136 WRITE: 136 137 OPEN: 1576 1576 OPEN_CONFIRM: 1 6 ACCESS: 161 1169 (savings of 86%) GETATTR: 628 903 (savings of 30%) LOOKUP: 496 1494 (savings of 67%) GET_DIR_DELEG: 7 DELEGRETURN: 1
Doing make(1) on the 2.6.16 linux kernel (first with directory delegations, then without):
READ: 13692 13588 WRITE: 18828 18701 OPEN: 496285 496286 ACCESS: 7400 11582 (savings of 36%) GETATTR: 23398 30345 (savings of 23%) LOOKUP: 21814 33947 (savings of 36%) READDIR: 883 1014 (savings of 13%) GET_DIR_DELEG: 749 DELEGRETURN: none
