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We are dumping a 4TB postgres database using pg_dump. The source is on a copy-on-write snapshot filesystem, and the target is an external drive.

While the pg_dump is running we get about 4-5MB/sec writes on the filesystem where postgresql data is (iostat), and specifically inside the base/ directory.

Since the only query running on this DB is the COPY initiated from pg_dump which only reads data, who is writing data to postgres datafiles?

We need to find a way to make postgres stop writing because the source copy-on-write filesystem gets filled up before the dump is finished.

Postgresql version is 8.4, vacuum is not running (autovacuum = off).

UPDATE: I recompiled pg_dump to use READ UNCOMMITTED transactions, but that didn't change anything.

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When experiencing this, do you have any files in the pg_xlog directory? –  dezso Dec 2 '13 at 16:47
    
xlog dir is not empty. This was good thinking, but after 3 days of pg_dump and 1TB of data modified in the base/ directory I wouldn't think it's the xlog that causes the writes. Moreover there are no reads on the xlog partition (it's a different partition so I can measure easily). –  sivann Dec 3 '13 at 15:04
    
The (exact) call might reveal something ... –  Erwin Brandstetter Feb 1 at 0:14
    
@ErwinBrandstetter it's just a pg_dump, no special options. –  sivann Feb 2 at 11:38

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