Re: Usage of epoch in txid_current

On 2019-Jun-22, Alexander Korotkov wrote:

> 2. Also introduce FullMultixactId, and apply to MultixactId the
> similar change as #1.
> 3. Change SLRU on-disk format to handle 64-bit xids and multixacts.
> In particular make SLRU page numbers 64-bit too. Add SLRU upgrade
> procedure to pg_upgrade.

I think enlarging multixacts to 64 bits is a terrible idea. I would
rather get rid of multixacts completely; zheap is proposing not to use
multixacts at all, for example. The amount of bloat caused by
pg_multixact data is already pretty bad ... because of which requiring
pg_multixact to be rewritten by pg_upgrade would cause a severe slowdown
for upgrades. (It worked for FSM because the volume is tiny, but that's
not the case for multixact.)

I think the pg_upgrade problem can be worked around by creating a new
dir pg_multixact64 (an example) which is populated from the upgrade
point onwards; so you keep the old data unchanged, and new multixacts
use the new location, but the system knows to read the old one when
reading old tuples. But, as I said above, I would rather not have
multixacts at all.

Another idea: create a new table AM that mimics heapam (I think ß-heap
"eszett-heap" is a good name), except that it reuses zheap's idea of
keeping "transaction locks" separately for tuple locks rather than
multixacts; heapam continues to use 32bits multixact. Tables can be
migrated from heapam to ß-heap (alter table ... set access method) to
incrementally reduce reliance on multixacts going forward. No need for
pg_upgrade-time disruption.

--
Álvaro Herrera https://www.2ndQuadrant.com/
PostgreSQL Development, 24x7 Support, Remote DBA, Training & Services