[DRBD-user] How efficient is DRBD during Sync?

Lars Ellenberg lars.ellenberg at linbit.com
Thu Oct 22 16:50:14 CEST 2015

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, Oct 22, 2015 at 01:03:10AM -0400, Digimer wrote:
> On 21/10/15 03:57 PM, Christian Völker wrote:
> > Hi all,
> > 
> > just a simple question for my own curiosity.
> > 
> > Assume a two node master-slave setup. Disk on the master fails. All
> > reads and writes go through the slave (master remains primary).
> > 
> > Now the disk on the primary is replaced and the full sync is started.
> > 
> > Now let's say the sync is at 40% and a write occurs which fits into the
> > not yet synced 60% of the disk.
> > 
> > Ist the block now written in parallel to both nodes and marked on the
> > primary as "already synced" or does the write affect only the slave and
> > during the remaining sync the block is copied back to the primary?
> > 
> > Just curious....
> > 
> > Thanks!
> > 
> > Chrischan
> Once connected, all new writes are synchronous, yes. Out of sync blocks
> replicate at the resync rate. This is why setting a high resync rate
> will slow down applications writing to DRBD because the bandwidth
> available for new writes is total capacity minus capacity used by resync.

But yes, any write during resync to no-yet synced, "dirty" blocks
will bring those blocks in sync as well.

As an extreme example, if you'd just zero-out the full device during
resync, you'd bring it in-sync by application writes.

: Lars Ellenberg
: http://www.LINBIT.com | Your Way to High Availability
: DRBD, Linux-HA  and  Pacemaker support and consulting

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