Re: Moving pg_xlog

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@interactive.co.uk> 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