[DRBD-user] mount seconday read-write

Lars Ellenberg lars.ellenberg at linbit.com
Thu Jun 28 18:05:05 CEST 2007

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

On Thu, Jun 28, 2007 at 05:03:46PM +0200, H.D. wrote:
> On 28.06.2007 16:52, Mike Tewner wrote:
> >Is it possible to use a non-cluster filesystem such as ext3 if you mount
> >both drives primary, one of them read-only?
> No, but you are free to take a snapshot of the secondary DRBD device and 
> mount that.
> That way you can have your secondary box do backups without the primary 
> even notice -- given you run ACID conform applications like a DBMS.

"without the primary even notice" ...
uhm, well, it will notice something.

you obviously propagate some performance penalty back to the primary,
since every primary write not yet in cow-area now causes a syncronous
read -> copy -> write transaction for the snapshot...

whether or not this is noticable depends very much on the actual
workload, and on the lower level io subsystem.

it may hurt very badly if you happen to have your snap cow area on the
same spindle as the origin volume, since you then basically send your
secondaries io subsystem thrashing... so don't.

make sure that your snap cow area is on some other spindle (other pv).

: Lars Ellenberg                            Tel +43-1-8178292-0  :
: LINBIT Information Technologies GmbH      Fax +43-1-8178292-82 :
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