[DRBD-user] syncing progresses even if devices are of different size?

Lars Ellenberg lars.ellenberg at linbit.com
Thu Jan 10 02:14:06 CET 2008

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

On Wed, Jan 09, 2008 at 04:44:51PM +0100, Tomasz Chmielewski wrote:
> I just noticed that it is possible to sync a bigger device to a smaller 
> device (i.e., drbd will sync a 500 GB device onto a 450 GB device).
> drbd_receiver.c will even warn us of a "considerable difference", but 
> the sync will progress:
> /* warn if the arguments differ by more than 12.5% */
> static void warn_if_differ_considerably(struct drbd_conf *mdev,
>         const char *s, sector_t a, sector_t b)
> {
>         sector_t d;
>         if (a == 0 || b == 0)
>                 return;
>         d = (a > b) ? (a - b) : (b - a);
>         if ( d > (a>>3) || d > (b>>3))
>                 WARN("Considerable difference in %s: %llus vs. %llus\n", s,
>                      (unsigned long long)a, (unsigned long long)b);
> }
> What's the point in trying to synchronize a bigger device on a smaller one?
> Just asking, perhaps there are some valid uses.

cat /proc/partitions

if you configure drbd, and for some reason have one node with 450GB,
the other node with 500GB, then drbd will of course know that it can
only use the minimum.

mainly its just convenience for you, we don't force you to configure
exactly identically sized backing store.
differences of some sectors or even tracks can be seen often.  that is
why drbd only warns you when it thinks you maybe did not mean to waste
those 50GB. but if you want to, its up to you.
maybe you have two vgs with different pe size, and the older does not
align with the newer one, you introduce drbd after the fact etc...

: Lars Ellenberg                           http://www.linbit.com :
: DRBD/HA support and consulting             sales at linbit.com :
: LINBIT Information Technologies GmbH      Tel +43-1-8178292-0  :
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