[DRBD-user] writer order on secondary site

Lars Ellenberg lars.ellenberg at linbit.com
Fri Jan 8 21:39:25 CET 2016

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

On Sun, Dec 27, 2015 at 01:26:15PM +0800, Mia Lueng wrote:
> How does epoch works? I have examined the source code.  when primary
> recieves a bio with bi_size=0 or  bio num > MAX_EPOCH_SIZE or idle
> time exceeds limit , there will be a P_BARRIER packet triggered?

Yes, that too.
But the important one is:

> > Basic idea is: whenever we complete something to upper layers,
> > we bump an atomic epoch counter.
> > Anything sumitted within one epoch cannot possibly depend on
> > something else within that same epoch.
> > So within such an epoch, reordering is allowed.
> > "on the wire", epochs are separated by "DRBD barriers".
> > Receiving side uses drain/flush/barrier (depending on configuration)
> > to make sure anything from a previous epoch is on stable storage
> > before starting to submit for the next epoch.

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

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