[DRBD-user] How Long Can a Primary Stay Disconnected from a Secondary?

Florian Haas florian.haas at linbit.com
Thu Mar 3 18:09:55 CET 2011

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


On 2011-03-03 16:13, Digimer wrote:
> On 03/03/2011 10:05 AM, Robinson, Eric wrote:
>> I am thinking about taking a secondary node offline for maintenance
>> during production hours. I assume it cannot stay offline forever because
>> eventually the primary would run out of space to record changes that
>> need to be sync'd. Is there a way to know in advance how long it is safe
>> to stay disconnected without resync'ing?
> 
> As I understand it, dirty blocks are recorded in memory. I am sure there
> is an upper limit to the counters, but I would hazard a guess that you
> would run out of RAM first.

Gaaah, we'd never be crash safe if we recorded this in memory only. One
node fails, we track the sync, the other node fails, boom the sync
information is gone. No, DRBD is a wee bit smarter than that. :)

You can overwrite your whole device while disconnected. DRBD will simply
eventually have flipped all the bits in the bitmap. Which it keeps on
stable storage, obviously.

Florian

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