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
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 Inder,                                    0131 229 1052 / 07808 492 213
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