[DRBD-user] DRBD Question

Diego Julian Remolina diego.remolina at ibb.gatech.edu
Wed Jan 10 13:24:36 CET 2007


>> However, I can argue that if one machine can take the load, then it
>> would be a waste of money to  have two machines and one of them there
>> sitting all the time doing nothing, just waiting.
> 
> No, it is not a waste of money.  If you believe it is a waste of money,
> then frankly you aren't looking at what drbd is for.
> 

If your master never goes down, then your slave has never done anything other than being a backup. 
So two machines there, one working, one doing nothing but waiting, and waiting and waiting. If you 
split things between both machines, then you have both machines doing work and the other one being a 
backup, but they are both being used, not one used and one waiting. That is my point. I consider a 
waste of money to have a very expensive machine sitting there doing nothing while I could have split 
services and have two machines doing something and have a live backup (drbd).

>> I do not currently have an active/active setup, but have DRBD 0.7.x
>> running on two machines with two  drbd partitions. One machine is mail
>> master (sendmail+mimedefang+spamassassin+clamav+cyrus-imap) and  the
>> other machine is web master (apache+php+mysql). Since we do not get that
>> much traffic in our web  server and mail server, I am sure one machine
>> would be able to take both loads simultaneously and  have lots of power
>> to spare. They are very decent machines hardware wise (Dual Opteron 270
>> with 4GB  Ram, Areca SATA raid controllers in RAID 10 and Gbit links to
>> our campus network).
> 
> If you are sure one machine would be able to take both loads, your
> argument is not logical.  It does not matter, economically, if you use one
> machine and have another acting as purely a drbd slave.  You have not
> "saved" anything.  You still have two machines.  
> 

It matters from the point of view of utilization. You are utilizing both machines actively, not just 
utilizing one actively and one passively (backup or hot spare machine with drbd and heartbeat). We 
can argue about this forever and everyone may see it differently; to me, if I can have both doing 
something, then it is better than have one master doing everything and one "hot spare".

> Again, since you claim one can easily take the load of both, then you in
> the precise same position as both machines sharing the load.
> 
> The only way you can "save", is by having both machines loaded, so that
> services slow down when one machine dies.  In such a case, you are not
> truly redundant.  Worse, your language above admits that you just think
> that one machine can handle the load, you have not actually empirically
> tested it.
> 

Of course I have tested it, remember kernel upgrades? or drbd upgrades? You have to take
one machine down to boot the new kernel. I keep my machines always with the latest versions of 
kernel and drbd, so I do cycle them periodically.

> Right now, you're not really redundant.  When was the last time you pulled
> the plug from your drbd master, and let the slave run for a few hours?

Like I've already said, I have done this for maintenance.

Diego


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