----- Original Message -----
From: Chander Ganesan
> DRBD is a synchronous method of transferring data - it's not asynchronous.
Assuming you are using Protocol C you should find that both are always in
sync. If you are using protocol B then things will be in sync so long as
the remote system didn't crash, and if you are using Protocol A then things
should be in sync as long as the local system didn't crash.
Apologies if you thought I was insinuating DRBD is categorically
asynchronous.
But you assumed wrong, we use protocol A, because we don't have a fast WAN
link < 10Mbps.
According to the DRBD documentation:
Protocol A: write IO is reported as completed, if it has reached local disk
and local tcp send buffer.
Which I interpret as: A for Asynchronous, which means I can complete writing
data and start writing the next set of data before the first set of data has
been synchronised at the remote site.
This is the cost of our system - we are willing to loose data for improved
performance of our master server. We've had one failure in 7 years that I've
been at the company... Many thanks to Linux and more importantly PostgreSQL!
One day when our local telecomms company stop the monopoly on leased data
lines, we may upgrade our DR link to a 100Mbps or even 1Gbps link, in which
case, protocol B or C will be a better solution. Unfortunately I think
there's a better chance that the cost of postage will go down first.
