DRBD performance question ?

Alan Robertson alanr at unix.sh
Tue Mar 5 14:07:33 MST 2002


Ravi Wijayaratne wrote:

> Hi,
> 
> I was looking at DRBD performance numbers from 
> http://www.complang.tuwien.ac.at/reisner/drbd/performance.html
> 
>>From Phillip Reisner's paper I gather that protocol A
> is an asynchronous protocol. Therefore protocol A
> should not severely impact the write performance at
> the
> primary server. Is the above assertion correct ?
> 
> If it is so how is it that the performance of protocol
> A on the primary side is seems to be limitted by the
> network B/W ? If protocol A is asynchronous we should
> see a significant difference in throughput between
> protocol A and C. However they seem to be quite close.
> Is this discrepency caused by write ordering or is
> there a hidden bottleneck in the protocol ?


First of all I'm sure that these performance numbers (from a year ago) were 
based on 2.2 kernels.  In 2.2, DRBD had to suffer the disk I/O scheduling 
twice: one at the DRBD level, and one at the real disk layer.  So, with 2.4 
(where this is avoided), the numbers look a lot different.

As an aside, disks don't write that much compared to their total bandwidth, 
and that the smart protocol only rarely has to synchronize.

A dedicated 100mbit connection (which is what he tested with) provides about 
8 megabytes/second writes.  If you say each write is a 2kbyte block, then 
that's nearly 4k block writes per second.  Commonly read rates significantly 
  outweigh write rates.  I have measured 10 to 1 ratios on general purpose 
development systems.  That would mean that it would have to be doing 44k 
block I/Os per second, which is a VERY amazing speed for most PC-based I/O 
subsystems.

If you have a very fast disk device like a ramdisk, or an expensive RAID 
controller with battery-backed up write cache, I would expect to see DRBD 
impact performance some - even in 2.4 kernels.

	-- Alan Robertson
	   alanr at unix.sh




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