[CentOS] Help setting up multipathing on CentOS 4.7 to an Equallogic iSCSI target

James Pearson james-p at moving-picture.com
Wed Apr 1 13:10:03 UTC 2009


nate wrote:
>>I thought if it treated each ip as a separate target portal on the
>>initiator you would be able to connect to two "different" targets at
>>the same time and let dm-multipath figure out it's the same disk. No?
> 
> 
> You can do this, I'm sure of it. But the catch is this doesn't
> aggregate the links, even if you use round robin multipathing at
> any particular instant in time your only using one link.
> 
>>From the docs from device mapper multipath on CentOS 5.2
> 
> Path Group:
>         A grouping of paths. With DM-MP, only one path group--the
>         active path group--receives I/O at any time. Within a path
>         group, DM-MP selects which ready path should receive I/O in a
>         round robin fashion. Path groups can be in various states (refer to
>         "Path Group States").
> 
> So as far as I can see you can't aggregate paths in CentOS 5.2
> multipath either for a single volume. The only way to use more
> than one path is to have more than one volume. You could set it
> up as active/passive and have each volume "prefer" a different
> path, or use round-robin and just know that at some points
> in time both volumes will be going down the same path.
> 
> I suppose you could aggregate the volumes themselves using LVM
> or software RAID, to present a single file system to the OS that
> uses more than one path simultaneously depending on what data is
> being accessed.
> 
> I think you'll probably find the software iSCSI has more serious
> performance bottlenecks before your able to max out a 1Gbps link
> anyways.

Just to update this thread with what I've discovered:

Using CentOS 5.2, the software iSCSI initiator and DM multipath, you can 
get more than 1Gbps to/from an Equallogic array using a single host and 
a single volume on the array.

I followed the instructions at:

<http://linfrastructure.blogspot.com/2008/02/multipath-and-equallogic-iscsi.html>

and with a multi-threaded read/write test I got 175Mbyte/s writes and 
230Mbyte/s reads using 2 GbE NICs

The Equallogic does some sort of connection load balancing, as you point 
  each iSCSI initiator on the host to the same IP address on the array 
(what they term the 'group IP address).

The software iSCSI initiator with CentOS 4.7 doesn't support multiple 
NICs to a single target, but using a couple of Qlogic 4060 iSCSI HBAs, 
DM multipath also works - although the same read and write tests were 
both 175Mbyte/s

Also, out of interest, using a different a iSCSI target array and a 10 
GbE network with the software iSCSI initiator on 5.2, I could get over 
400Mbyte/s on both read and writes ...

James Pearson


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