[dm-devel] dm-multipath has great throughput but we'd like more!

[Bob Gautier]

On Thu, 2006-05-18 at 02:25 -0500, Jonathan E Brassow wrote:
> The system bus isn't a limiting factor is it?  64-bit PCI-X will get 
> 8.5 GB/s (plenty), but 32-bit PCI 33MHz got 133MB/s.
> 
> Can your disks sustain that much bandwidth? 10 striped drives might get 
> better than 200MB/s if done right, I suppose.
> 
> Don't the switches run at 2 Gbits/s?  2 Gbits/s / 10 (throw in 2 bits 
> for protocol) ~= 200MB/s.
> 

Thanks for the fast responses:

The card is a 64-bit PCI-X, so I don't think the bus is the bottleneck,
and anyway the vendor specifies a maximum throughput of 200Mbyte/s per
card.

The disk array does not appear to be the bottleneck because we get
200Mbyte/s when we use *two* HBAs in load-balanced mode.

The question is really about why we only see O(100Mbyte/s) with one HBA
when we can achieve O(200MByte/s) with two cards, given that one card
should be able to achieve that throughput.

I don't think the method of producing the traffic (bonnie++ or something
else) should be relevant but if it were that would be very interesting
for the benchmark authors!

The storage is an HDS 9980 (I think?)

> Could be a bunch of reasons...
> 
>   brassow
> 
> On May 18, 2006, at 2:05 AM, Bob Gautier wrote:
> 
> > Yesterday my client was testing of multipath load balancing and 
> > failover
> > on a system running ext3 on a logical volume which comprises about ten
> > SAN LUNs all reached using multipath in multibus mode over two QL2340
> > HBAs.
> >
> > On the one hand, the client is very impressed: running bonnie++
> > (inspired by Ronan's GFS v VxFS example) we get just over 200Mbyte/s
> > over the two HBAs, and when we pull a link we get about 120MByte/s.
> >
> > The throughput and failover response times are better than the client
> > has ever seen, but we're wondering why we are not seeing higher
> > throughput per-HBA -- the QL2340 datasheet says it should manage
> > 200Mbyte/s and all switches etc. run at 2GBps.
> >
> > Any ideas?
> >
> > Bob Gautier
> > +44 7921 700996
> >
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>