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@xxxxxxxxxxx> 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@xxxxxxxxxxxxxxxxxxx> wrote: > >> On Mon, 2011-05-02 at 22:04 -0700, Malahal Naineni wrote: > >>> John A. Sullivan III [jsullivan@xxxxxxxxxxxxxxxxxxx] 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 > >> > >> -- > >> dm-devel mailing list > >> dm-devel@xxxxxxxxxx > >> https://www.redhat.com/mailman/listinfo/dm-devel > >> > > > > -- > dm-devel mailing list > dm-devel@xxxxxxxxxx > https://www.redhat.com/mailman/listinfo/dm-devel -- dm-devel mailing list dm-devel@xxxxxxxxxx https://www.redhat.com/mailman/listinfo/dm-devel
