N/A wrote:
>Chris,
>
>The NAS is a Network Appliance 920c. I understand that an individual file
>while be "intact" during a snap, (not sure I believe it, but that's what I'm
>told).
>
Depending on the way that data is journalled, this may be a possibility
(istr XFS is able to do this).
> The NAS may be able to monitor the state of an individual file and
>not "snap" it if it is on an "open" state or has a pending write, or some
>such, but as the NAS has no knowledge of groups of files that need to exist
>in a known state, (it deals on the file level), it cannot guarantee a
>consistent state across a group of files.
>
That is a serious concern. Every table in PostgreSQL consists of at
least one physical file. Transactions may touch several of these
files. While it is not impossible to solve this problem (as I
mentioned, I think XFS has this capability), it is hardly trivial or
something you would normally expect.
Also I wonder what the impact of several Postmasters trying to access
data over NFS would be.
Best Wishes,
Chris Travers