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Re: [dm-devel] Shell Scripts or Arbitrary Priority Callouts?

On Fri, Mar 20, 2009 at 06:01:23AM -0400, John A. Sullivan III wrote:
> > 
> > John:
> > 
> > Thanks for the reply.
> > 
> > I ended up writing a small C program to do the priority computation for me.
> > 
> > I have two sets of FC-AL shelves attached to two dual-channel Qlogic
> > cards. That gives me two paths to each disk. I have about 56 spindles
> > in the current configuration, and am tying them together with md
> > software raid.
> > 
> > Now, even though each disk says it handles concurrent I/O on each
> > port, my testing indicates that throughput suffers when using multibus
> > by about 1/2 (from ~60 MB/sec sustained I/O with failover to 35 MB/sec
> > when using multibus).
> > 
> > However, with failover, I am effectively using only one channel on
> > each card. With my custom priority callout, I more or less match the
> > disks with even numbers to the even numbered scsi channels with a
> > higher priority. Same with the odd numbered disks and odd numbered
> > channels. The odds are 2ndary on even and vice versa. It seems to work
> > rather well, and appears to spread the load nicely.
> > 
> > Thanks again for your help!
> > 
> I'm really glad you brought up the performance problem. I had posted
> about it a few days ago but it seems to have gotten lost.  We are really
> struggling with performance issues when attempting to combine multiple
> paths (in the case of multipath to one big target) or targets (in the
> case of software RAID0 across several targets) rather than using, in
> effect, JBODs.  In our case, we are using iSCSI.
> Like you, we found that using multibus caused almost a linear drop in
> performance.  Round robin across two paths was half as much as aggregate
> throughput to two separate disks, four paths, one fourth.
> We also tried striping across the targets with software RAID0 combined
> with failover multipath - roughly the same effect.
> We really don't want to be forced to treated SAN attached disks as
> JDOBs.  Has anyone cracked this problem of using them in either multibus
> or RAID0 so we can present them as a single device to the OS and still
> load balance multiple paths.  This is a HUGE problem for us so any help
> is greatly appreciated.  Thanks- John


Hmm.. just a guess, but could this be related to the fact that if your paths
to the storage are different iSCSI sessions (open-iscsi _doesn't_ support
multiple connections per session aka MC/s), then there is a separate SCSI
command queue per path.. and if SCSI requests are split across those queues 
they can get out-of-order and that causes performance drop?


Especially the reply from Ross (CC). Maybe he has some comments :) 

-- Pasi

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