Re: [DOCS] Needs discussion of pg_xlog
"Joshua D. Drake" writes: > On 12/01/2016 07:00 AM, rob...@interactive.co.uk wrote: >> The only mention of this that I've seen is in Section 29.5 (WAL >> Internals), >> and that just says "it is advantageous...", with no explanation. > The reason it can be advantageous is that pg_xlog has a different write > profile that $PGDATA. The WAL is written sequentially versus randomly. Yeah. The traditional understanding of that was you wanted to keep a write head positioned over the current end-of-WAL, which of course only applies to spinning rust. It's still true that under heavy update loads, your I/O volume to WAL is probably comparable to your I/O volume to everything else, which might justify a separate SSD just on write bandwidth grounds. But seek delays aren't part of the calculation anymore. regards, tom lane -- Sent via pgsql-docs mailing list (pgsql-docs@postgresql.org) To make changes to your subscription: http://www.postgresql.org/mailpref/pgsql-docs
Re: [DOCS] Needs discussion of pg_xlog
On 12/01/2016 07:00 AM, rob...@interactive.co.uk wrote: The following documentation comment has been logged on the website: Page: https://www.postgresql.org/docs/9.4/static/runtime-config-file-locations.html Description: In various places (e.g. stackoverflow), I've seen people suggest having pg_xlog ion a different disk (i.e. not in /var/lib/pgsql/9.X/data). The only mention of this that I've seen is in Section 29.5 (WAL Internals), and that just says "it is advantageous...", with no explanation. If this IS a good idea, then "Server Configuration / File Locations" is where it should be discussed: in particular, why is it a good idea, and how does it affect backups (and in particular base backups to a standby server). The reason it can be advantageous is that pg_xlog has a different write profile that $PGDATA. The WAL is written sequentially versus randomly. JD -- Command Prompt, Inc. http://the.postgres.company/ +1-503-667-4564 PostgreSQL Centered full stack support, consulting and development. Everyone appreciates your honesty, until you are honest with them. Unless otherwise stated, opinions are my own. -- Sent via pgsql-docs mailing list (pgsql-docs@postgresql.org) To make changes to your subscription: http://www.postgresql.org/mailpref/pgsql-docs
[DOCS] Needs discussion of pg_xlog
The following documentation comment has been logged on the website: Page: https://www.postgresql.org/docs/9.4/static/runtime-config-file-locations.html Description: In various places (e.g. stackoverflow), I've seen people suggest having pg_xlog ion a different disk (i.e. not in /var/lib/pgsql/9.X/data). The only mention of this that I've seen is in Section 29.5 (WAL Internals), and that just says "it is advantageous...", with no explanation. If this IS a good idea, then "Server Configuration / File Locations" is where it should be discussed: in particular, why is it a good idea, and how does it affect backups (and in particular base backups to a standby server). -- Sent via pgsql-docs mailing list (pgsql-docs@postgresql.org) To make changes to your subscription: http://www.postgresql.org/mailpref/pgsql-docs
