Since its introduction in fc34b0d9de, the default
maintenance_io_concurrency has been larger than the default
effective_io_concurrency. maintenance_io_concurrency primarily
controlled prefetching done on behalf of the whole system, for
operations like recovery. Therefore it makes sense for it to have a
value equal to or greater than effective_io_concurrency, which controls
I/O concurrency for reading a relation in a bitmap heap scan.
ff79b5b2ab increased effective_io_concurrency to 16, so we'll increase
maintenance_io_concurrency as well. For now, though, we'll keep the
defaults of effective_io_concurrency and maintenance_io_concurrency
equal to one another (16).
On fast, high IOPs systems, significantly higher values of
maintenance_io_concurrency are observably beneficial [1]. However, such
values would flood low IOPs systems and increase overall system I/O
latency.
It is worth mentioning that since 9256822608 and c3e775e608,
maintenance_io_concurrency also controls the I/O concurrency of each
vacuum worker. Since many autovacuum workers may be simultaneously
issuing I/Os, we want to keep maintenance_io_concurrency appropriately
conservative.
[1] https://postgr.es/m/c5d52837-6256-0556-ac8c-d6d3d558820a%40enterprisedb.com
Suggested-by: Jakub Wartak <jakub.wartak@enterprisedb.com>
Discussion: https://postgr.es/m/CAKZiRmxdHQaU%2B2Zpe6d%3Dx%3D0vigJ1sfWwwVYLJAf%3Dud_wQ_VcUw%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
-----------------------------------------
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