[DRBD-user] Real live risk of data loss w/o flush

Robert Verspuy robert at exa-omicron.nl
Thu Sep 2 15:22:25 CEST 2010

Note: "permalinks" may not be as permanent as we would like,
direct links of old sources may well be a few messages off.

  On 08/09/2010 11:08 AM, Sebastian Hetze wrote:
> Hi *,
> What is your opinion and possibly your experience with using
> no-disk-barrier and no-disk-flushes without BBU RAID?  The reason for
> me asking is the huge latency I suffer using flushes in my setup
> where I run several virtual KVM instances in DRBD containers without
> BBU RAID. These virtual systems frequently flush disks and these
> operations occasionally queue up to a substantial epoch of 100 or even
> higher.

See also my other 2 messages to the list, mailed yesterday and today.
After some testing on our new database cluster,
I'm seeing a huge latency in writing small packets to disk with flushes.
Now I'm going to use protocol C, no-disk-barrier, no-disk-flushes, and 
no BBU on primary and secondary.

Your message helped me thinking about the risks.

Both our servers have 2 power supplies, connected to 2 power feeds.
So in case of a power failure of one feed, both servers will still be 

Just like you mention, only in a complete power failure in the datacentre,
drbd will loose data, but at that moment all other servers using the 
database server are also offline.

On the database server we're using PostgreSQL.
PostgreSQL is ACID-compliant, so the data on disk should not be corrupt.
It could be possible that we lost some database insert/updates,
but that's a risk I'm willing to accept, looking at the small change 
that all power is lost.

Robert Verspuy

Patroonsweg 10
3892 DB Zeewolde
Tel.: 088-OMICRON (66 427 66)
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