[DRBD-user] drbd performance with GbE in connected mode

Ross S. W. Walker rwalker at medallion.com
Sun Jan 14 23:40:30 CET 2007


> -----Original Message-----
> From: drbd-user-bounces at lists.linbit.com 
> [mailto:drbd-user-bounces at lists.linbit.com] On Behalf Of Ralf Gross
> Sent: Sunday, January 14, 2007 5:33 PM
> To: drbd-user at lists.linbit.com
> Subject: Re: [DRBD-user] drbd performance with GbE in connected mode
> 
> Ross S. W. Walker schrieb:
> >> [drbd write performance over GbE]
> > 
> > First of all let me ask which benchmark tool you used for the
> > sequential/random io runs? I like the information it provides and
> > can use a tool like that.
> 
> It's tiobench (http://sourceforge.net/projects/tiobench/).
>  
> > Second, using Prot C, the writes do not return until they are
> > committed to disk on both sides, so cannot the loss of throughput be
> > attributed to the increased latency of writing on host1 while it
> > sends out the write over 1Gbps wire to host2, who then writes it to
> > disk and then sends a successful response back to host1 who then
> > returns?
> > 
> > If the local write and send happened at the same time Copy/Send
> > Latency + Gbps latency + Remote Write Latency + Gbps return latency
> > = increased latency. How much increased latency depends on the
> > speed/effectiveness of the network and the speed of the remote
> > storage versus the speed of the local storage. The slowest storage
> > is always the weakest link, try to get storage systems that are the
> > same for most reliable performance.
> 
> Both Raid's are able to write with >120MB/s in disconnected mode.
> I'm just wondering because I have seen people on the list that get
> ~95MB/s with a similar setup. 

There's lies, damn lies, then benchmarks. Pump a big enough block size
to the storage till you get the throughput you like. Some people throw
1MB block size and get 115MB/s throughput.

I always like to use the block sizes my application uses. If it's ext2
then 4K is a good benchmark, but different apps use different block
sizes.

Always ask the block size and io pattern for the benchmarks people throw
around.

-Ross

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