[dm-devel] [Multipath] Round-robin performance limit

Mon Oct 24 11:50:21 UTC 2011

On Sat, Oct 22, 2011 at 06:02:47PM +0300, Pasi Kärkkäinen wrote:
> On Wed, Oct 05, 2011 at 03:54:35PM -0400, Adam Chasen wrote:
> > John,
> > I am limited in a similar fashion. I would much prefer to use multibus
> > multipath, but was unable to achieve bandwidth which would exceed a
> > single link even though it was spread over the 4 available links. Were
> > you able to gain even a similar performance of the RAID0 setup with
> > the multibus multipath?
> > 
> 
> Utilizing multiple links works with for example this setup:
> - VMware ESXi 4.1 software iSCSI initiator.
> - Dell Equallogic iSCSI target.
> 
> The steps needed for ESXi are:
> - Configure multiple VMkernel (vmkX) IP interfaces.

And I forgot to write this:
- Bind the vmkX interfaces to portgroups that use dedicated NICs.

> - Configure ESXi iscsi initiator to use (bind to) all the vmkX interfaces.
> - Configure the path selection policy to be RR (RoundRobin).
> - Configure multipath to switch paths after 3 IOs.
> 
> 
> The same should work with Linux dm-multipath.
> 
> 

That should be it.

-- Pasi

> 
> > Thanks,
> > Adam
> > 
> > On Tue, Oct 4, 2011 at 11:07 PM, John A. Sullivan III
> > <jsullivan at opensourcedevel.com> wrote:
> > > On Tue, 2011-10-04 at 16:19 -0400, Adam Chasen wrote:
> > >> Unfortunately even with playing around with various settings, queues,
> > >> and other techniques, I was never able to exceed the bandwidth of more
> > >> than one of the Ethernet links when accessing a single multipathed
> > >> LUN.
> > >>
> > >> When communicating with two different multipathed LUNs, which present
> > >> as two different multipath devices, I can saturate two links, but it
> > >> is still a one to one ratio of multipath devices to link saturation.
> > >>
> > >> After further research on multipathing, it appears people are using md
> > >> raid to achieve multipathed devices. My initial testing of using raid0
> > >> md-raid device produces the behavior I expect of multipathed devices.
> > >> I can easily saturate both links during read operations.
> > >>
> > >> I feel using md-raid is a less elegant solution than using
> > >> dm-multipath, but it will have to suffice until someone can provide me
> > >> some additional guidance.
> > >>
> > >> Thanks,
> > >> Adam
> > > We recently changed from the RAID0 approach to multipath multibus.
> > > RAID0 did seem to give more even performance over a variety of IO
> > > patterns but it had a critical flaw.  We could not use the snapshot
> > > capabilities of the SAN because we could never be certain of
> > > snapshotting the RAID0 disks in a transactionally consistent state.  If
> > > I have four disk in a RAID0 array and snapshot them all, how can I be
> > > assured that I have not done something like written two of three stripes
> > > and no parity.  This was our singular reason for discarding RAID0 over
> > > iSCSI for multipath multibus - John
> > >
> > >>
> > >> On Mon, Oct 3, 2011 at 11:08 PM, Adam Chasen <adam at chasen.name> wrote:
> > >> > Malahal,
> > >> > After your mentioning bio vs request based I attempted to determine if
> > >> > my kernel contains the request based mpath. It seems in 2.6.31 all
> > >> > mpath was switched to request based. I have a kernel 2.6.31+ (actually
> > >> > .35 and .38), so I believe I have requrest-based mpath.
> > >> >
> > >> > All,
> > >> > There also appears to be a new multipath configuration option
> > >> > documented in the RHEL 6 beta documentation:
> > >> > rr_min_io_rq    Specifies the number of I/O requests to route to a path
> > >> > before switching to the next path in the current path group, using
> > >> > request-based device-mapper-multipath. This setting should be used on
> > >> > systems running current kernels. On systems running kernels older than
> > >> > 2.6.31, use rr_min_io. The default value is 1.
> > >> >
> > >> > http://docs.redhat.com/docs/en-US/Red_Hat_Enterprise_Linux/6-Beta/html/DM_Multipath/config_file_multipath.html
> > >> >
> > >> > I have not tested using this setting vs rr_min_io yet or even if my
> > >> > system supports the configuration directive.
> > >> >
> > >> > If I trust some of the claims of several VMware ESX iscsi multipath
> > >> > setups, it is possible (possibly using different software) to gain a
> > >> > multiplicative throughput by adding additional Ethernet links. This
> > >> > makes me hopeful that we can do this with open-iscsi and dm-mulitpath
> > >> > as well.
> > >> >
> > >> > It could be something obvious I am missing, but it appears a lot of
> > >> > people experience this same issue.
> > >> >
> > >> > Thanks,
> > >> > Adam
> > >> >
> > >> > On Tue, May 3, 2011 at 6:12 AM, John A. Sullivan III
> > >> > <jsullivan at opensourcedevel.com> wrote:
> > >> >> On Mon, 2011-05-02 at 22:04 -0700, Malahal Naineni wrote:
> > >> >>> John A. Sullivan III [jsullivan at opensourcedevel.com] wrote:
> > >> >>> > I'm also very curious about your findings on rr_min_io.  I cannot find
> > >> >>> > my benchmarks but we tested various settings heavily.  I do not recall
> > >> >>> > if we saw more even scaling with 10 or 100.  I remember being surprised
> > >> >>> > that performance with it set to 1 was poor.  I would have thought that,
> > >> >>> > in a bonded environment, changing paths per iSCSI command would give
> > >> >>> > optimal performance.  Can anyone explain why it does not?
> > >> >>>
> > >> >>> rr_min_io of 1 will give poor performance if your multipath kernel
> > >> >>> module doesn't support request based multipath. In those BIO based
> > >> >>> multipath, multipath receives 4KB requests. Such requests can't be
> > >> >>> coalesced if they are sent on different paths.
> > >> >> <snip>
> > >> >> Ah, that makes perfect sense and why 3 seems to be the magic number in
> > >> >> Linux (4000 / 1460 (or whatever IP payload is)).  Does that change with
> > >> >> Jumbo frames? In fact, how would that be optimized in Linux?
> > >> >>
> > >> >> 9KB seems to be a reasonable common jumbo frame value for various
> > >> >> vendors and that should contain two pages but, I would guess, Linux
> > >> >> can't utilize it as each block must be independently acknowledged. Is
> > >> >> that correct? Thus a frame size of a little over 4KB would be optimal
> > >> >> for Linux?
> > >> >>
> > >> >> Would that mean that rr_min_io of 1 would become optimal? However, if
> > >> >> each block needs to be acknowledged before the next is sent, I would
> > >> >> think we are still latency bound, i.e., even if I can send four requests
> > >> >> down four separate paths, I cannot send the second until the first has
> > >> >> been acknowledged and since I can easily place four packets on the same
> > >> >> path within the latency period of four packets, multibus gives me
> > >> >> absolutely no performance advantage for a single iSCSI stream and only
> > >> >> proves useful as I start multiplexing multiple iSCSI streams.
> > >> >>
> > >> >> Is that analysis correct? If so, what constitutes a separate iSCSI
> > >> >> stream? Are two separate file requests from the same file systems to the
> > >> >> same iSCSI device considered two iSCSI streams and thus can be
> > >> >> multiplexed and benefit from multipath or are they considered all part
> > >> >> of the same iSCSI stream? If they are considered one, do they become two
> > >> >> if they reside on different partitions and thus different file systems?
> > >> >> If not, then do we only see multibus performance gains between a single
> > >> >> file system host and a single iSCSI host when we use virtualization each
> > >> >> with their own iSCSI connection (as opposed to using iSCSI connections
> > >> >> in the underlying host and exposing them to the virtual machines as
> > >> >> local storage)?
> > >> >>
> > >> >> I hope I'm not hijacking this thread and realize I've asked some
> > >> >> convoluted questions but optimizing multibus through bonded links for
> > >> >> single large hosts is still a bit of a mystery to me.  Thanks - John
> > >> >>
> > >> >> --
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> > >> >> dm-devel at redhat.com
> > >> >> https://www.redhat.com/mailman/listinfo/dm-devel
> > >> >>
> > >> >
> > >>
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> > >
> > >
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> > 
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