nbtxlog.c:btree_xlog_vacuum() contains the following comment:
* XXX we don't actually need to read the block, we just need to * confirm it is unpinned. If we had a special call into the * buffer manager we could optimise this so that if the block is * not in shared_buffers we confirm it as unpinned.
The attached patch introduces an XLogLockBlockRangeForCleanup() function that addresses this, as well as a "special call into the buffer manager" that makes it possible to lock the buffers without reading them.
In GetBufferWithoutRelcache(), I was wondering, rather than calling PinBuffer(), if we do this :
LockBufHdr(buf);
PinBuffer_Locked(buf);
valid = ((buf->flags & BM_VALID) != 0);
then we can avoid having the new buffer access strategy BAS_DISCARD that is introduced in this patch. And so the code changes in freelist.c would not be necessary.
Will give further thought on the overall logic in XLogLockBlockRangeForCleanup().
will follow up with some performance numbers soon.
