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Aggressive vacuums must scan every unfrozen tuple in order to advance the relfrozenxid/relminmxid. Because data is often vacuumed before it is old enough to require freezing, relations may build up a large backlog of pages that are set all-visible but not all-frozen in the visibility map. When an aggressive vacuum is triggered, all of these pages must be scanned. These pages have often been evicted from shared buffers and even from the kernel buffer cache. Thus, aggressive vacuums often incur large amounts of extra I/O at the expense of foreground workloads. To amortize the cost of aggressive vacuums, eagerly scan some all-visible but not all-frozen pages during normal vacuums. All-visible pages that are eagerly scanned and set all-frozen in the visibility map are counted as successful eager freezes and those not frozen are counted as failed eager freezes. If too many eager scans fail in a row, eager scanning is temporarily suspended until a later portion of the relation. The number of failures tolerated is configurable globally and per table. To effectively amortize aggressive vacuums, we cap the number of successes as well. Capping eager freeze successes also limits the amount of potentially wasted work if these pages are modified again before the next aggressive vacuum. Once we reach the maximum number of blocks successfully eager frozen, eager scanning is disabled for the remainder of the vacuum of the relation. Original design idea from Robert Haas, with enhancements from Andres Freund, Tomas Vondra, and me Reviewed-by: Robert Haas <robertmhaas@gmail.com> Reviewed-by: Masahiko Sawada <sawada.mshk@gmail.com> Reviewed-by: Andres Freund <andres@anarazel.de> Reviewed-by: Robert Treat <rob@xzilla.net> Reviewed-by: Bilal Yavuz <byavuz81@gmail.com> Discussion: https://postgr.es/m/flat/CAAKRu_ZF_KCzZuOrPrOqjGVe8iRVWEAJSpzMgRQs%3D5-v84cXUg%40mail.gmail.com
<!-- doc/src/sgml/README.non-ASCII -->
Representation of non-ASCII characters
--------------------------------------
Find non-ASCII characters using:
grep --recursive --color='auto' -P '[\x80-\xFF]' .
Convert to HTML4 named entity (&) escapes
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We support several output formats:
* html (supports all Unicode characters)
* man (supports all Unicode characters)
* pdf (supports only Latin-1 characters)
* info
While some output formatting tools support all Unicode characters,
others only support Latin-1 characters. Specifically, the PDF rendering
engine can only display Latin-1 characters; non-Latin-1 Unicode
characters are displayed as "###".
Therefore, in the SGML files, we only use Latin-1 characters. We
typically encode these characters as HTML entities, e.g., Álvaro.
It is also possible to safely represent Latin-1 characters in UTF8
encoding for all output formats.
Do not use UTF numeric character escapes (&#nnn;).
HTML entities
official: http://www.w3.org/TR/html4/sgml/entities.html
one page: http://www.zipcon.net/~swhite/docs/computers/browsers/entities_page.html
other lists: http://www.zipcon.net/~swhite/docs/computers/browsers/entities.html
http://www.zipcon.net/~swhite/docs/computers/browsers/entities_page.html
https://en.wikipedia.org/wiki/List_of_XML_and_HTML_character_entity_references