As a followup on this - I've tracked down the problem, and wanted to say thanks to all the people offering insight - most of it moved me in the right direction, which I'm summarising here because it's at least a little interesting.

It boils down to this - on the reading host, my pcap looks like:

79542  10.643148 10.0.0.52 -> 10.0.0.24 NFS 222  ACCESS allowed   testfile  V3 ACCESS Call, FH: 0x76a9a83d, [Check: RD MD XT XE]
79543  10.643286 10.0.0.24 -> 10.0.0.52 NFS 194 0 ACCESS allowed 0600 Regular File testfile NFS3_OK V3 ACCESS Reply (Call In 79542), [Allowed: RD MD XT XE]
79544  10.643335 10.0.0.52 -> 10.0.0.24 NFS 222  ACCESS allowed     V3 ACCESS Call, FH: 0xe0e7db45, [Check: RD LU MD XT DL]
79545  10.643456 10.0.0.24 -> 10.0.0.52 NFS 194 0 ACCESS allowed 0755 Directory  NFS3_OK V3 ACCESS Reply (Call In 79544), [Allowed: RD LU MD XT DL]
79546  10.643487 10.0.0.52 -> 10.0.0.24 NFS 230  LOOKUP    testfile  V3 LOOKUP Call, DH: 0xe0e7db45/testfile
79547  10.643632 10.0.0.24 -> 10.0.0.52 NFS 190 0 LOOKUP  0755 Directory  NFS3ERR_NOENT V3 LOOKUP Reply (Call In 79546) Error: NFS3ERR_NOENT
79548  10.643662 10.0.0.52 -> 10.0.0.24 NFS 230  LOOKUP    testfile  V3 LOOKUP Call, DH: 0xe0e7db45/testfile
79549  10.643814 10.0.0.24 -> 10.0.0.52 NFS 190 0 LOOKUP  0755 Directory  NFS3ERR_NOENT V3 LOOKUP Reply (Call In 79548) Error: NFS3ERR_NOENT

On my writing host - I get:

203306  13.805489  10.0.0.6 -> 10.0.0.24 NFS 246  LOOKUP    .nfs00000000d59701e500001030  V3 LOOKUP Call, DH: 0xe0e7db45/.nfs00000000d59701e500001030
203307  13.805687 10.0.0.24 -> 10.0.0.6  NFS 186 0 LOOKUP  0755 Directory  NFS3ERR_NOENT V3 LOOKUP Reply (Call In 203306) Error: NFS3ERR_NOENT
203308  13.805711  10.0.0.6 -> 10.0.0.24 NFS 306  RENAME    testfile,.nfs00000000d59701e500001030  V3 RENAME Call, From DH: 0xe0e7db45/testfile To DH: 0xe0e7db45/.nfs00000000d59701e500001030
203309  13.805982 10.0.0.24 -> 10.0.0.6  NFS 330 0,0 RENAME  0755,0755 Directory,Directory  NFS3_OK V3 RENAME Reply (Call In 203308)
203310  13.806008  10.0.0.6 -> 10.0.0.24 NFS 294  RENAME    testfile_temp,testfile  V3 RENAME Call, From DH: 0xe0e7db45/testfile_temp To DH: 0xe0e7db45/testfile
203311  13.806254 10.0.0.24 -> 10.0.0.6  NFS 330 0,0 RENAME  0755,0755 Directory,Directory  NFS3_OK V3 RENAME Reply (Call In 203310)
203312  13.806297  10.0.0.6 -> 10.0.0.24 NFS 246  CREATE    testfile_temp  V3 CREATE Call, DH: 0xe0e7db45/testfile_temp Mode: EXCLUSIVE
203313  13.806538 10.0.0.24 -> 10.0.0.6  NFS 354 0,0 CREATE  0755,0755 Regular File,Directory testfile_temp NFS3_OK V3 CREATE Reply (Call In 203312)
203314  13.806560  10.0.0.6 -> 10.0.0.24 NFS 246  SETATTR  0600  testfile_temp  V3 SETATTR Call, FH: 0x4b69a46a
203315  13.806767 10.0.0.24 -> 10.0.0.6  NFS 214 0 SETATTR  0600 Regular File testfile_temp NFS3_OK V3 SETATTR Reply (Call In 203314)

(IPs modified).

The long and short of it is this - that _most_ of the time, everything works right, but when the file that's being overwritten (and deleted) has otherwise been opened for reading by another process - two RENAME operations occur, because NFS preserves the 'deleted' as part of it's stateless protocol thing - it has to be valid to open and unlink a file, and continue to be able to do IO to it, and that's how NFS solves the problem.

RENAME remains atomic from the client perspective, but you have a teeny tiny race condition between the two renames, during which the remote client might get NFS3ERR_NOENT (And return NOENT to the client) because there isn't a file present.

The reason I had a hard time reproducing this is it simply doesn't happen in a simplisitic 'single writer' scenario - and doesn't happen often in our environment, because it _also_ requires the file to be open (for reading) at the same time. Addition of a pretty simple 'open file; sleep 1;' type loop was what caused it to occur more reliably/repeatably.

Net result though, is that NFS doesn't offer any guarantees of atomicity of rename to remote clients - only to the client performing the rename operation.

On 28 December 2016 at 11:21, Edward Rolison <ed.rolison@gmail.com> wrote:

Hello fellow NetApp Admins.
I have a bit of an odd one that I'm trying to troubleshoot - and whilst I'm not sure it's specifically filer related, it's NFS related (and is happening on a filer mount).

What happens is this - there's a process that updates a file, and relies on 'rename()' being atomic- a journal is updated, and then reference pointer (file) is newly created, and renamed over an old one.

The expectation is that this file will always be there - because "rename()" is defined as an atomic operation.

But that's not quite what I'm getting - I have one nfs client doing it's (atomic) rename. And another client (different NFS host) reading it, and - occasionally - reporting 'no such file or directory'.

This is causing an operation to fail, which in turn means that someone has to intervene in the process. This operation (and multiple extremely similar ones) happen at 5m intervals, and every few days (once a week maybe?) it fails for this reason, and our developers think that should be impossible. But as such - it looks like a pretty narrow race condition.

So what I'm trying to figure out is first off:

- Could this be a NetApp bug? We've moved from 7 mode to CDOT, and it didn't happen before. On the flip side though - I have no guarantee that it 'never happened before' because we weren't catching a race condition. (moving to new tin and improving performance does increase race condition likelihood after all)

- Could this be a kernel bug? We're all on kernel 2.6.32-504.12.2.el6.x86_64 - and whilst we're deploying Centos 7, all the hosts involved aren't yet. (But that's potentially also just coincidence, as there's quite a few hosts, and they're all the same kernel versions).

- Is it actually impossible for a file A renamed over file B to generate ENOENT on a different client? Specifically, in RFC3530 We have: " The RENAME operation must be atomic to the client.". So the client doing the rename sees an atomic operation - but the expectation is that a separate client will also perceive an 'atomic' change - once the cache is refreshed, the 'new' directory has the new files, and at no point was there 'no such file or directory' because it was either the old one, or the newly renamed one. Is this actually a valid thing to think?

This is a bit of a complicated one, and has me clutching at straws a bit - I can't reliably reproduce it - a basic fast spinning loop script on multiple client to read-write-rename didn't hit it. I've got pcaps running hoping to catch it 'in flight' - but haven't yet managed to catch it happening. But any suggestions would be gratefully received.