Re: Dead Space Map version 3 (simplified)

ITAGAKI Takahiro wrote:
> Attached is an updated DSM patch. I've left the core function of DSM only
> and dropped other complicated features in this release.

We discussed it a long time ago already, but I really wished the DSM
wouldn't need a fixed size shared memory area. It's one more thing the
DBA needs to tune manually. It also means we need to have an algorithm
for deciding what to keep in the DSM and what to leave out. And I don't
see a good way to extend the current approach to implement the
index-only-scans that we've been talking about, and the same goes for
recovery. :(

The way you update the DSM is quite interesting. When a page is dirtied,
the BM_DSM_DIRTY flag is set in the buffer descriptor. The corresponding
bit in the DSM is set lazily in FlushBuffer whenever BM_DSM_DIRTY is
set. That's a clever way to avoid contention on updates. But does it
work for tables that have a small hot part that's updated very
frequently? That's exactly the scenario where the DSM is the most
useful. Hot pages stay in the buffer cache because they're frequently
accessed, which means that FlushBuffer isn't getting called for them and
the bits in the DSM aren't getting set until checkpoint. This could lead
to unnecessary bloating of the hot part. A straightforward fix would be
to scan the buffer cache for buffers marked with BM_DSM_DIRTY to update
the DSM before starting the vacuum scan.

It might not be a problem in practice, but it bothers me that the DSM
isn't 100% accurate. You end up having a page with dead tuples on it
marked as non-dirty in the DSM at least when a page is vacuumed but
there's some RECENTLY_DEAD tuples on it that become dead later on. There
might be other scenarios as well.

If I'm reading the code correctly, DSM makes no attempt to keep the
chunks ordered by block number. If that's the case, vacuum needs to be
modified because it currently relies on the fact that blocks are scanned
and the dead tuple list is therefore populated in order.

--
Heikki Linnakangas
EnterpriseDB http://www.enterprisedb.com