Thanks, everyone, for your comments. I think I've got a clearer idea of what's going on now... Robert. On 1 December 2016 at 13:55, Robert Inder <robert@xxxxxxxxxxxxxxxxx> wrote: > 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 -- 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
