Re: pg_basebackup bug: base backup is double the size of the database

[Craig James <cjames@xxxxxxxxxxxxxx>]

One clarification:

On Wed, Jan 21, 2015 at 9:32 AM, Craig James <cjames@xxxxxxxxxxxxxx> wrote:

We've encountered a serious bug with pg_basebackup. It seems to be following hard links and duplicating all files in the tablespaces rather than preserving links.

It could be barman, not pg_basebackup, that has the bug.  I assumed that barman was using pg_basebackup, but one of my colleagues pointed out that we're using barman, which may or may not use pg_basebackup to do its work.

Craig

 

Drilling down into one specific tablespace, we find this:

# ls -l /data/postgres-9.3/main/pg_tblspc/16747

lrwxrwxrwx 1 postgres postgres 27 2014-08-18 11:28 /data/postgres-9.3/main/pg_tblspc/16747 -> /postgres/tablespaces/uorsy/

# du -sh /data/postgres-9.3/tablespaces/uorsy

35G     /data/postgres-9.3/tablespaces/uorsy

# du -sh /data/postgres-9.3/tablespaces/uorsy/*

35G     /data/postgres-9.3/tablespaces/uorsy/8208624

8.1M    /data/postgres-9.3/tablespaces/uorsy/PG_9.3_201306121

4.0K    /data/postgres-9.3/tablespaces/uorsy/pgsql_tmp

4.0K    /data/postgres-9.3/tablespaces/uorsy/PG_VERSION

# find /data/postgres-9.3/tablespaces/uorsy \! -links 1 -type f | wc -l

740

In other words, this tablespace has 35G of real data, plus 740 hard links that effectively duplicate each data file.

When we look at the same data in the archive that pg_basebackup creates (invoked via barman), we find this:

# du -sh /pg_archive/staging/base/20150114T170002/pgdata/pg_tblspc/16747

70G     /pg_archive/staging/base/20150114T170002/pgdata/pg_tblspc/16747

# du -sh /pg_archive/staging/base/20150114T170002/pgdata/pg_tblspc/16747/*

35G     /pg_archive/staging/base/20150114T170002/pgdata/pg_tblspc/16747/8208624

35G     /pg_archive/staging/base/20150114T170002/pgdata/pg_tblspc/16747/PG_9.3_201306121

4.0K    /pg_archive/staging/base/20150114T170002/pgdata/pg_tblspc/16747/pgsql_tmp

4.0K    /pg_archive/staging/base/20150114T170002/pgdata/pg_tblspc/16747/PG_VERSION

# find /pg_archive/staging/base/20150114T170002/pgdata/pg_tblspc/16747 \! -links 1 -type f | wc -l

0

That is, no hard links, and all of the data files are duplicated. And of course, when we try to actually use this archive to recover, it's twice the size as the original database and doesn't fit on our disks.

My guess is that pg_basebackup is using (or doing the equivalent of) rsync(1) without the --hard-links option, and that these hard links were created by pg_upgrade when we went from 8.4.17 to 9.3.5.

What can we do to fix this? The whole cluster is about 350 databases and 800GB.

Thanks,

Craig

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Craig A. James

Chief Technology Officer

eMolecules, Inc.

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