Thread: XLogFlush

Maybe this is one of those things that is obvious when someone points
it out to you, but right now I am not seeing it.  If you look at the
last eight lines of this snippet from XLogFlush, you see that if we
obtain WriteRqstPtr under the WALInsertLock, then we both write and
flush up to the highest write request.  But if we obtain it under the
info_lck, then we write up to the highest write request but flush only
up to our own records flush request.  Why the disparate treatment?
The effect of this seems to be that when WALInsertLock is busy, group
commits are suppressed.

if (LWLockConditionalAcquire(WALInsertLock, LW_EXCLUSIVE)){    XLogCtlInsert *Insert = &XLogCtl->Insert;    uint32
 freespace = INSERT_FREESPACE(Insert);        if (freespace < SizeOfXLogRecord)        /* buffer is full */
WriteRqstPtr= XLogCtl->xlblocks[Insert->curridx];    else    {        WriteRqstPtr =
XLogCtl->xlblocks[Insert->curridx];       WriteRqstPtr.xrecoff -= freespace;    }    LWLockRelease(WALInsertLock);
WriteRqst.Write= WriteRqstPtr;    WriteRqst.Flush = WriteRqstPtr;}else{    WriteRqst.Write = WriteRqstPtr;
WriteRqst.Flush= record;}
 

Cheers,

Jeff

Jeff Janes <jeff.janes@gmail.com> writes:
> Maybe this is one of those things that is obvious when someone points
> it out to you, but right now I am not seeing it.  If you look at the
> last eight lines of this snippet from XLogFlush, you see that if we
> obtain WriteRqstPtr under the WALInsertLock, then we both write and
> flush up to the highest write request.  But if we obtain it under the
> info_lck, then we write up to the highest write request but flush only
> up to our own records flush request.  Why the disparate treatment?

I think the point of the check within the info_lck section is that the
global Write pointer must not be allowed to go backward.  It's likely
unnecessary though, since there is probably a defense against that
in XLogWrite (or if not there should be).

The other bit of the reasoning that doesn't seem well commented is
that if we can't find out what the global status is (because of failure
to acquire the insert lock), we should just do the work we know we need,
not guess at some greater requirement.
        regards, tom lane