"Mr. Dan" <bitsandbytes88@hotmail.com> writes:
>> How are you doing the copies, exactly? SQL COPY command, psql \copy,
>> something else?
> We've tried SQL COY and psql \copy and always get random results - 0,1, or 2
> blocks of 25 rows missing.
Hmph. If it happens with a SQL COPY command then psql seems to be off
the hook, and that also eliminates some theories about dropped TCP
packets and such.
Would you check back in the source table for the COPY and see what the
ctid values are for the missing rows? I'm wondering about a pattern
like "the dropped rows of a group are all on the same disk page", ie,
what's being missed is one whole page at a time.
If that's what's happening, the only very plausible theory I can think
of is that your disk drive is sometimes glitching and returning a page
of all-zeroes instead of what it should return. Postgres will not
complain about this in normal operation (because there are legitimate
error-recovery scenarios where a zero page can be in a table); it'll
just treat the page as empty. VACUUM will complain though, so the next
step would be to set up a test table by copying your large table and
then repeatedly run plain VACUUM on the test table. If you get sporadic
warnings "relation foo page N is uninitialized --- fixing" then we have
the smoking gun. Don't run this test directly on a valuable table, as
each such message would mean you just lost another page of data :-(
FWIW, I spent several hours yesterday evening copying 6GB tables around
to see if I could reproduce any such problem, and I couldn't...
regards, tom lane