[OpenAFS] Cache Consistency problem

[Craig_Everhart@transarc.com]

Excerpts from transarc.external.openafs: 17-Dec-02 Re: [OpenAFS] Cache Co=
nsist.. Paul Blackburn@est.ibm.c (5759*)

Ka=F0an Co=FEkun wrote:

> Hi All,
>
> We are having some cache consistency problems. Sometimes the afs =

> client on our web server does not represent the actual files on the =

> some volumes. That is when someone uploads a new file to his directory =

> the file is not noticed on the client side. The volume is a read/write =

> volume. And the problem can be overcomed by creating a file on the =

> volume using the client and also flush commands work too.


AFS (and OpenAFS) provides open/close consistency, not read/write consist=
ency.  After a file is written and closed, a new open() on any client wil=
l see the fresh data.  The mechanism in use here is the callback that Pau=
l Blackburn mentioned.  The problem is likely that the web server is cach=
ing the opens on the files that it is serving, so it is not seeing (servi=
ng) the fresh data.  If the web server shares a client cache with the upd=
ater, they both will see the same data, as you noticed.  So basically the=
 web server is keeping a cache of open files that is overriding the usual=
 AFS mechanism for automatically noticing the fresh data.  By contrast, D=
CE/DFS provides read/write consistency, so after a write() completes, a r=
ead() on any client retrieves fresh data.

		Craig