[OpenAFS] Anyone seen this weirdness...

[Jeffrey Altman]

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Robert Banz wrote:
>>
>> Could you do some rxdebug calls to the fileserver next time? So we
>> know why it's getting unresponsive.
>> It could be running out of threads. I don't expect that, but it could
>> be ...
> 
> The 'symptoms' seem to be, for the most part, volume-specific.  Slow
> response to accessing that volume, followed by the clients seeing a
> timeout on it.  So, guts-o-the-fileserver folk, is there a volume-wide
> lock that gets set by a particular fileserver thread when it's being
> acted upon?  Since deleting a whole-bunch-of-files (a /bin/rm -fr <dir>)
> is happening, that's a whole lot of requests coming in in-series to that
> volume, being taken care of on (probably) a first-come, first-served
> basis, leaving little room for other clients to get an op in on that
> volume?

Lets say that there are N clients who are all attempting to use
the contents of directory "D" in the read-write volume "V".  Client 1
is making changes to the contents of the directory and clients 2 to N
are reading the directory.

In order for each of the N clients to read the directory, they need
to perform a FetchData RPC which registers a callback with the file
server on "D".  Now each time that client 1 makes a change to the
contents of "D", each of the callbacks that are currently registered
with the file server must be broken in order to notify clients 2 to N
that the data they have cached is no longer valid.  If clients 2 to N
are actively using "D", then when the callback break is received from
the file server they will in turn attempt to perform a new FetchData
operation to obtain the latest data value.  This in turn registers a
new callback.

Now if client 1 is performing 30,000 individual RemoveFile RPCs it is
going to extremely hard for any of the other clients to maintain to be
able to maintain a callback until all 30,000 RPCs are completed.  As
soon as a FetchData operation completes, the callback will be broken
and the cache contents will be invalidated.

I don't believe there is a bug here its just a negative side effect
of client side caching and the fact that the file system is only given
one file name at a time to act on.

Jeffrey Altman

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