I'm running Postgres9.4 in master/hot-standby mode on a few pairs of servers. While recovering from A Bit Of Bother last week, I came across a posting saying that pg_xlog should be on a separate partition. I tried to find out more about this, by consulting the PostgresQL documentation (i.e. https://www.postgresql.org/docs/9.4/static/index.html ) But all I could find was a mention that "It is advantageous if the log is located on a different disk from the main database files". The questions: 1. WHY is this good? Is it (just) to stop pg_xlog filling the database disk/partition? Or are there performance implications? SPECIFICALLY: my database is currently in "/", which is on SSD. Is it better to move pg_xlog to another partition on the same SSD? Or to a physical disk or SAN? 2. What are the implications for doing a base backup? I believe I read that putting pg_xlog on a different partition meant it would be omitted from a file-system bulk copy (e.g. rsync), and this was a GOOD thing, because the copy operation would be faster -- not copying pg_xlog would not prevent the standby server from starting, because the information it needed would be in the WAL files that would be shipped separately. Have I got that right? Finally, the suggestion. I'd really like to read an explicit discussion of this in the official documentation, rather than just glean what I can from answers to questions. The possibility of moving pg_xlog to another disk is mentioned in the documentation, but I almost missed it because it is in "the wrong place". It is in Section 29.5 -- "Reliability and the Write Ahead Log" / "WAL Internals". But I wasn't interested in anything INTERNAL: I wanted to know where I should try to locate it/them. So I'd looked in "the obvious places" -- Section 18 (Server configuration), and in particular 18.2 "File Locations". Could I suggest that the motivation for doing this, and the consequences for backups, should be discussed in "the right place" -- in or near the section that talks about file locations in the context of server configuration. Robert. -- Robert Inder, 0131 229 1052 / 07808 492 213 Interactive Information Ltd, 3, Lauriston Gardens, Edinburgh EH3 9HH Registered in Scotland, Company no. SC 150689 Interactions speak louder than words -- Sent via pgsql-general mailing list (pgsql-general@xxxxxxxxxxxxxx) To make changes to your subscription: http://www.postgresql.org/mailpref/pgsql-general
