[DRBD-user] DRBD performance on a budget

Christian Iversen chrivers at iversen-net.dk
Tue Sep 15 00:48:02 CEST 2009

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

Hey DRBD-list

I'm trying to build a reasonably performant DRBD-minicluster (2 machines 
with heartbeat). I've got the basic setup working fine, with 2 largely 
identical servers each running a Seagate 7200.ES 500GB disk.

This is exported as a HA-NFS on top of the 500GB DRBD volume with 
internal metadata. It's used in our production system, and we're 
generally very happy with the result (failover, etc), except the 
performance leaves a bit to be desired.

I have a few questions:

1) It really seems internal metadata is a bad idea, since it eats 2 
extra seeks per write. Is this always true, or can DRBD sometimes buffer 
these metadata-writes to minimize impact?

2) Suppose write latency (related to question #1) is our biggest 
concern. We are currently using protocol C. Will protocol A or B be 
vastly different? I know there's no substitute for a proper 
benchmarking, but we are a little wary of "just testing something" on 
our production system. Does anybody have experience with this they would 
like to share?

3) Suppose the write latency really is the biggest problem we're facing 
(we're still investigating the situation). As far as I can tell, a 
battery-backed storage controller is the way to go, since this will 
allow persistent stores to complete right away. However, this is for a 
normal server, not DRBD. How does DRBD affect this situation? Will such 
a controller even be useful at all?

4) If it will be useful, can anybody recommend a good SATA battery 
backed controller?

I look forward to hearing any comments you might have.

P.S: I should note that we are not willing to sacrifice data integrity 
for performance. If it seems that way from my comments about protocol 
A/B/C, let me assure you that we simply want to learn about the various 
bottlenecks in the system.

Med venlig hilsen
Christian Iversen

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