Re: 2 HBA's and Multipath.

[SoulBlazer <soul@lamp-post.net>]

What about write balancing ?

I would also be interested in discussing a possible patch to the raid code to 
do write/read balancing over N hba's if it is that trivial.

On April 8, 2002 03:16 pm, Luca Berra wrote:
> On Mon, Apr 08, 2002 at 02:02:56PM -0500, SoulBlazer wrote:
> > Question,
> >
> > I have 2 Qlogic Fiberchannel HBA's (2100F's) connected directly to both
> > FCAL ports on my Sun A5200 22 Disk disk array.  If I enable multipath, do
> > I get twice the throughput between my linux box and the array ? Or is
> > multipath purely for failover.
> >
> > Additionally do I get twice the bandwidth anyhow since both HBA's see the
> > same disk ?  How can I tell if either one or the other or both hba's are
> > being utilized...
>
> multipath does not do any read-balancing (see code below),
> while i don't think it would be difficult to implement (cut&paste from
> raid1.c) i did not have any time to try
>
> only way i found to do read-balancing at the moment is if you have multiple
> luns on your array you can create md configs with alternating channel
> order.
>
> regards,
> Luca
>
> ------- from 2.4.18-rc1/drivers/md/multipath.c
> static int multipath_read_balance (multipath_conf_t *conf)
> {
>         int disk;
>
>         for (disk = 0; disk < conf->raid_disks; disk++)
>                 if (conf->multipaths[disk].operational)
>                         return disk;
>         BUG();
>         return 0;
> }