Re: Dead Space Map version 3 (simplified)

This needs additional changes for memory mangement and we don't have
time to do that for 8.3, Sorry:

This has been saved for the 8.4 release:

http://momjian.postgresql.org/cgi-bin/pgpatches_hold

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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.
>
> VACUUM finishs faster with the patch, but it's obvious... DSM vacuum
> sweeps only pages that have many dead tuples and leave some of them
> after vacuum.
>
> I'll examine the sweep behavior and the performance from now.
>
>
> * Features
> - DSM tracks pages worth vacuuming using 1bit/page bit.
> The threshold is two dead tuples or 2kB of deadspaces.
> - DSM is constructed at page flush. Almost of the works are done by
> bgwriter if it is properly configured.
> - 'VACUUM' command uses DSM. 'VACUUM ALL' always scans all pages.
> - This is including n_dead_tuples statistics fix.
> http://momjian.us/mhonarc/patches/msg00002.html
>
> * Configuration
> - max_dsm_relations (=1000)
> Counterpart to max_fsm_relations, but count tables only;
> Indexes are not tracked by DSM.
> - max_dsm_pages (=1024000)
> Counterpart to max_dsm_pages. Default values are configurated to
> 5 times of max_fsm_pages at initdb.
> - min_dsm_target (=8MB)
> Minimum size of tables of which dead space is tracked
> to avoid tracking small tables, including system catalogs.
>
> * Limitation
> - XID-wraparound vacuum is still required. VACUUM with DSM cannot
> update relfrozenxid, so we sometimes needs full-scan.
> - No recovery support. All contents of DSM and FSM are lost on crash.
> - DSM uses fixed size memory allocated at server start. We cannot change
> the value on-the-fly. If we want the feature, we need something like
> shared-memory-allocator or swap-supported memory management module.
>
> Regards,
> ---
> ITAGAKI Takahiro
> NTT Open Source Software Center

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Bruce Momjian <bruce(at)momjian(dot)us> http://momjian.us
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