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This is a new setup of ESXi 4.0 running VMs off of a Cybernetics miSAN D iSCSI SAN.

Doing a high data read test on a VM, it took 8 minutes vs 1.5 minutes on the the same VM located on a slower VMWare Server 1.0 Host with the VMs located on local disk. I'm watching my read speeds from the SAN, and it's getting just over 3MB/s max read, and Disk Usage on the VM matches at just over 3MB/s....horribly slow.

The server and SAN are both connected to the same 1GB Switch. I have followed this guide ith-vmware-vsphere.html

to get multipathing setup properly, but I'm still not getting good performance with my VMs. I know the SAN and the network should be able to handle over 100MB/s, but I'm just not getting it. I have two GB NICs on the SAN multipathed to two GB NICs on the ESXi Host. One NIC per VMkernel. Is there something else I can check or do to improve my speed? Thanks in advance for any tips.

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Did you ever figure this out? We are having similar issues. – prestomation Feb 8 '10 at 18:40

That SAN hardware is certified for Vmware so get your support to look into it. Common causes for bad performance are overload the interface of the SAN hardware, because if you have multiple connections to the same SAN not all can be served at the maximum speed.

Also your local disk will always be faster than your SAN in your setup, because even a SATA disk will have a maximum of 3Gb/s bandwidth, so your SAN will never match the speed of your local disks. You probably are also using ethernet instead of fibre which is also not help performance.

You use a SAN not only because of the speed, but to have a central managed place where you can put all your importante data and make sure a suitable RAID level is being applied. There are also certain features like replication which is one of the advantages of having a SAN.

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"You probably are also using ethernet instead of fibre which is also not help performance." I'm not sure this is always the case. If you have a dedicated ethernet network for the SAN with decent switches, and bonded links to your SAN array and hosts, you may get performance better than some fibre channel solutions. – dunxd Jan 26 '10 at 12:29
@echobeach2: No, not with a 1Gb/s pipe in the middle. – Scott Lundberg Jan 26 '10 at 14:46

That set up should be able to deliver reasonable performance and from what I can gather that array can sustain around 60-70Megabytes per sec even for small block random IO. I've no experience with them but the spec indicates that it should easily be able to handle your requirements and the few review that searches throw up back that up.

Anyway if I were you I'd step back a little first. Get rid of multipathing (initially) and make sure you can get a single path (on the VMware side) to sustain respectable performance. Assuming you have an 8 drive unit, fully populated with 10k SAS drives, one hot spare and have a 7 drive RAID 5 pack it should be able to easily deliver >100Meg/sec sequential read or write over a single interface on a good,dedicated Gbit LAN even accounting for all ip\tcp and iSCSI overhead. Do simple bulk tests of large file copies (something significantly larger than the write cache on the array) to or from the SAN to check that you are seeing that. If you are reading and writing to the SAN volume then performance will be no more than half that BTW. If not then you will want to look at all the usual suspects:

  • For starters make sure the SAN's cache is configured correctly
  • Make sure all the drives are healthy - ie you're not fighting a RAID rebuild
  • Make sure the switch is healthy and not busy with other stuff - ideally you should isolate your SAN traffic onto its own switch, if you can't do that put it on its own VLAN.
  • Definitely don't put it on a cheap switch that is very busy with other stuff.
  • Check duplex and speed settings on all the ports (ESX, Switch & SAN)
  • Avoid messing with Jumbo Frames and ESX until you know everything else is working
  • Definitely enable hardware flow control on the switch

When you are testing make sure neither the ESX host or the SAN is busy with anything else.

Once you are successfully getting >100Meg/sec for sequential traffic on a single uplink then you can consider seeing if multipathing makes a difference. With iSCSI on ESX4 it can but it's unlikely unless the storage array correctly supports it in conjunction with ESX 4- I would look to the array vendor for guidance on that.

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Multipathing may be causing your issue. Are you able, and have you tried, to disable multipathing and just have one 1Gb connection to your SAN? VMware may be path thrashing when put under load because of a bad link, or a delay in packet delivery...

BTW, your maximum throughput with a 1Gb link is going to be ~30MBytes/sec if your SAN and ESXi host were the only two devices on that link...

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Even accounting for ethernet packet overhead (18 bytes), inter packet gap (equivalent to ~12 bytes), preamble (7 bytes) ip overhead (20 bytes), TCP overhead (20 bytes) and iSCSI encapsulation overhead (48 bytes) with a standard (1500 byte payload) packet you are only talking about 8% overhead so 115Megabytes/sec is possible with the write streaming IO pattern. Under average conditions things will be worse but the maximum is way higher than 30MBytes/sec with good hardware. – Helvick Jan 27 '10 at 22:59
@Helvick: Yes, you are correct, but you also have to account for the time it takes for the OS to develop the packet, draining the TCP window, etc. Here is a link showing the relationship between latency and throughput: – Scott Lundberg Jan 28 '10 at 16:41

Which load-balancing policy are you using for your iSCSI NICs - in almost every situation one device talking to another device - each using any of the forms of bonding/etherchannel/teaming/whatever will only ever use one of however many links are available to talk - i.e. never more than 1Gbps, which by the way is almost never fully available, especially with iSCSI due to all the encapsulation involved.

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Do keep in mind that the native Multi-Pathing IO driver (MPIO) in VMware is Active/Passive only so it'll only use one path per LUN. So if all your traffic is going to a single LUN, you'll only use one path to get that traffic there. The only support 3rd party MPIO driver (that I know of) is EMC's PowerPath which is an Active/Active MPIO driver, but it requires the Enterprise Plus edition of vSphere.

Some things to look at.

Have you enabled Jumbo Frames on the SAN, switch and host? Is the SAN showing any performance problems via its monitoring tools? How many disks are sitting behind the LUN in question? How much other stuff is hitting those same disks?

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I believe VMware VSphere supports Active/Active MPIO, with its software iSCSI Initiator. At least my setup is using both paths concurrently. – Martijn Heemels Apr 13 '10 at 16:25

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