Re: xlog.c: WALInsertLock vs. WALWriteLock

On Fri, Oct 22, 2010 at 3:08 PM, fazool mein <fazoolmein@gmail.com> wrote:
> I'm writing a function that will read data from the buffer in xlog (i.e.
> from XLogCtl->pages and XLogCtl->xlblocks). I want to make sure that I am
> doing it correctly.
> For reading from the buffer, do I need to lock WALInsertLock or
> WALWriteLock? Also, can you explain a bit the usage of 'LW_SHARED'. Can we
> use it for read purposes?

Holding WALInsertLock in shared mode prevents other processes from
inserting WAL, or in other words it keeps the "end" position from
moving, while holding WALWriteLock in shared mode prevents other
processes from writing the WAL from the buffers out to the operating
system, or in other words it keeps the "start" position from moving.
So you could probably take WALInsertLock in shared mode, figure out
the current end of WAL position, release the lock; then take
WALWriteLock in shared mode, read any WAL before the end of WAL
position, and release the lock.  But note that this wouldn't guarantee
that you read all WAL as it's generated....

-- 
Robert Haas
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