[DRBD-user] Scheduled Replication Feasible?

Ross S. W. Walker rwalker at medallion.com
Fri Feb 9 03:58:05 CET 2007

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


I have posted on this site in the past about the feasibility of
replicating LVM snapshots and the short of it is, if I was willing to do
a full sync each time I created a snapshot to replicate, then yes it's
possible using an external meta-device.

Well that isn't really feasible in my environment, so I am abandoning
that idea.

I have thought about it some more and have come up for an idea to do
scheduled replications.

Here is what I have and what I need and maybe somebody can tell me if it
makes sense.

I have 2 storage arrays in 2 data centers about 2mi apart. They are
connected by only a T1 line.

Of course full-time synchronous replication would bring the array to a
crawl and fill up the T1. I then thought of asynchronous replication
using Prot A, but the size of the sndbuf would need to be really big,
and it would still hog the T1 during business hours.

I then thought, well why not do a bit of both. If I have the two arrays
setup to replicate using Prot A, but only run that replication after
hours, say from 8pm to 8am, at 8am I manually disconnect the arrays
putting the primary into standalone and the secondary into wait, then at
8pm I put the primary back into wait, so they connect and re-sync.

Question is, how long can the replicas stay disconnected before a
complete re-sync is necessary?

Can you tune this?

If we fail-over to the secondary which is in wait, will it be marked as
inconsistent and refuse to become primary or will it come up ok with the
previous data?

What would happen then if the original primary came back, a full-resync,
split-brain?

Does this even make sense?

Let me know, as this would work out well in our business as we aren't a
24/7 operation.


Ross S. W. Walker
Information Systems Manager
Medallion Financial, Corp.
437 Madison Avenue
38th Floor
New York, NY 10022
Tel: (212) 328-2165
Fax: (212) 328-2125
WWW: http://www.medallion.com <http://www.medallion.com> 



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