I'm not sure if this is the right forum for this question, so I'll start
here and escalate as necessary.

One of the things I do a lot is copy large tables from one database to
another. I'd started life using pgdump, but this requires I save the
whole table into an intermediate file (or play games with pipes).
Rather than doing either, I instead wrote a quick script which creates a
cursor at the source, and starts schlepping data from one to other. For
"small" tables (say, a million rows or less), this works just fine, but
when I try to copy a bigger table (tens of millions of rows), the
performance continually drops off, with the copy becoming slower and slower.

On 8.3-beta1, I see the memory utilization skyrocketing, specifically
the non-shared residential space of the postmaster daemon, driving the
whole system deep into swap (sooner or later), and bringing things to a
screeching halt. But on 8.1.4, I don't see memory utilization problem-
the memory utilization of a daemon pegs out at just a little larger than
the shared buffers size (i.e. what I'd expect)- but I still see the
slowdown.

It is definately the copy that is the problem- I've tried a variation of
my script with the copy commented out (just throwing the data away) to
test if it's the cursor- and I get 20K+ rows/second.from the cursor.
I've also tried breaking the copy up- finishing the current copy command
and starting a new one every million rows or so, and that doesn't help.
I've also fiddled with transactions, and rate-limiting the copy, and
neither of those help.

Is this a bug in postgres? If not, as I'm assume, what should I be
doing to make this work fast?

Brian

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  • Tom Lane at Oct 22, 2007 at 8:28 pm

    Brian Hurt writes:
    Is this a bug in postgres?
    Well, if you'd provide enough info for someone else to reproduce it, we
    could have a look.

    regards, tom lane
  • Brian Hurt at Nov 26, 2007 at 4:09 pm

    Tom Lane wrote:
    Brian Hurt <bhurt@janestcapital.com> writes:

    Is this a bug in postgres?
    Well, if you'd provide enough info for someone else to reproduce it, we
    could have a look.
    Hello- I just wanted to close the book on this problem. I first noticed
    the problem when long copies kept slowing down. It turns out it wasn't
    a problem with Postgresql at all, as I recreated it with bonnie++ (which
    explains the long silence on this issue). After doing a lot of
    sustained I/O, we see I/O wait times climb until we're getting virtually
    no I/O performance at all, and it doesn't matter if it's Postgresql or
    bonnie++ doing the I/O. iostat would report that we'd be doing only
    2MB/sec, and we'd be seeing 90%+ iowait percentages in atop or top. So
    our solution is to fix out I/O subsystem. We're replacing the expensive
    TLA SAN storage device with a low end Fibre Channel raid (which we were
    going to do anyways, as even at it's best, the TLA SAN wasn't
    impressive), upgrading from an old 2.4 linux kernel to a newer 2.6
    kernel, changing the I/O Scheduler to deadline, and upgrading the server
    hardware. Some combination of the above solves the problem.

    Hopefully no one has been lying awake worrying about this problem :-),
    but I wanted to close it out, and to leave a permanent record in the
    archives for the next person who has this problem.

    Brian

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postedOct 22, '07 at 7:44p
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