[DRBD-user] Poor fsync performance

[Andy Dills]

On Thu, 10 Jan 2013, Paul Freeman wrote:

> Andy,

> I see similar results for dd except the performance is even worse 
> (approx 150MByte/s (connected) vs 780MByte/s (disconnected).
> 
> It is possible you are seeing the same latency issue(s) I am and have 
> posted about on this list in the last few days.
> 
> title:[DRBD-user] High iowait on primary DRBD node with large sustained 
> writes and replication enabled to secondary
> 
> As Sebastian Riemer indicated in his comments on my blktrace analysis 
> there is latency between the primary and secondary drbd nodes when the 
> resource(s) is connected which does not exist when disconnected.

I'm new to drbd and still reading, so I'm confused about this page:

http://www.drbd.org/users-guide-8.3/s-throughput-overhead-expectations.html

If what that page says is true, well then of course we're being 
constrained by the bandwidth of our bonded connections.

But what's not clear, is if the contraint created by the network bandwidth 
is negated by doing this:

  disk {
    no-disk-barrier;    
    no-disk-flushes;
    no-md-flushes;
  }

I would think so? But I haven't yet gained full understanding of what 
disabling those safeguards implies; all I know is that since my RAIDs are 
BBU protected I should be using them.

Andy

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Andy Dills
Xecunet, Inc.
www.xecu.net
301-682-9972
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