[DRBD-user] Write order

LangTuSJ stanley.nguyen at gmail.com
Tue Jun 14 20:07:38 CEST 2011

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

Hi Lars,

Thanks very much for your reply.  I'm not sure if I understand your answer
to 2nd part of my question so allow me further clarify what I meant.

In case of DR, my primary server on Site 1 will be completely dead and
there's no way for me to access to it so I have to use what I currently have
on my secondary server on Site 2.  Now my main concern is the disaster
happened right in the middle of replicating.  DRBD was supposed to write 4
blocks of disk from Site 1 to Site 2 but it just completed 2 blocks.  If
this is using Protocol C, then I would not have any problem since it follows
"all-or-nothing" rule but I'm using Protocol A here.  My application on Site
2 wouldn't be started since it got incomplete fragment of data.  Unless DRBD
is smart enough to know that it gets disconnected in the middle and rolls
back that two blocks of disk, I'm very much in a messy state.  Does DRBD
provide a way to deal with this kind of out-of-sync, corrupted state?



> Another question is whether DRBD provides a recovery from an incomplete
> async replication.  Let's say my primary machine is in the process of
> replicating some data and it goes dead completely. Now my secondary
> machine
> is in a partially-replicated state.  Does DRBD give me an option to roll
> back to previous checkpoint?

If this was during normal operation ("Connected"), there is no need for
the replica is consistent as is, and the file system (or whatever else
lives on top of it) can do it's file system journal replay or any other
crash recovery procedure as usual.

> Kind of like all-or-nothing sync mode but for async replicating.

During a resync, of course, the SyncTarget is inconsistent.
You can use the "before-resync-target" handler to take a (lvm or
hardware raid box or san level) snapshot, and the "after-resync-target"
handler to release it again.

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

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