What is MPIO DSM/device mapper/NMP and why we need it?

I am writing this article to understand the basic functionality of Multipath I/O. This article is targeted at Beginners of SAN technology.

Why we need multipath I/O?

In early days all storage solutions were SCSI based and directly attached to the host. When Fibre channel was introduced it enabled us to provide storage access to multiple hosts from a single centralized storage system. But those days all storage systems were Active/Passive arrays so communication happens in one path.
Modern day storage systems are Symmetric and Asymmetric active active arrays. Now we need to understand the difference between these two in order to understand MPIO

Asymmetric Active/Active

Lets consider a dual controller/node storage system and a LUN is provisioned to host. Lets assume the LUN is owned by controller A and there is two active paths between host and array. Lets assume host is accessing this LUN via controller B. Since the LUN is owned by controller A it alone will get exclusive access to write or read data from the LUN. Controller B will communicate with other controller via its internal link to get the IO served.

Symmetric Active/Active

If its an Symmetric Active/Active array whoever is the owner of LUN any controller/nodes in the storage system will be able to read/write data on the LUN regardless of ownership.

Multipath I/O

Multipath IO is a feature will will effectively manage the active paths to storage array. There are various path selection polices(PSP) which can be set for individual LUN. Following are the commonly used PSPs

  1. Most Recently Used (MRU)
  2. Fixed
  3. Round Robin (RR)

Round Robin is the most commonly used path selection policy for various storage arrays due to its dynamic load sharing capability. In ESXi environments RR is preferred to achieve good performance.

Similarly the host software which is required to configure this has various names on different OS Platforms. Microsoft calls it as MPIO, in Linux world this host software is called as Device Mapper. Vmware goes by the name Native multipathing plug-in (NMP)

For optimum performance it is recommended to use device specific DSM files, here device meaning the storage array. Storage vendors will release DSM files for easier configuration and it will instruct the host is how to access the LUN using a set of predefined rules.

Apart from this, there is also wide range of software solutions to manage multipath for LUNs in a larger scale using softwares like EMC Powerpath, HP Secure Path, Hitachi HighCommand Dynamic Link Manager etc.

The best practice is to implement Multipath as recommended by the storage vendor. Selecting the appropriate storage specific PSP will lead to optimum performance of LUN.

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P2000 G3 LUN and vmware VAAI

This might probably known to many by now, recently I bumped into a problem with ESX 4.1 and ESX 5.1 hosts. I am SAN engineer and have limited expertise when it comes to vmware, the problem is when we try to add a lun presented from P2000 G3 MSA it failed with an error. Both hosts are blade server and are able to see the lun. It fails at a point when I try to add that lun as a datastore with vmfs5.

Error : Call "HostDatastoreSystem.CreateVmfsDatastore" for object "ha-datastoresystem" on ESX - "Unable to create filesystem: Connection timed out" Check the kernel log
HP released a customer advisory – http://h20000.www2.hp.com/bizsupport/TechSupport/Document.jsp?objectID=c03668075&lang=en&cc=us&taskId=135&prodSeriesId=5077022&prodTypeId=329290

As per HP HardwareAcceleratedLocking needs to be disabled, so let’s take some time to know about VAAI and what it really does?

VAAI – vStorage APIs for Array Integration

This feature was introduced in ESXi/ESX 4.1 It provides hardware acceleration for compatible storage hardware. That means host can intelligently manage its storage related operations and can reduce its CPU cycles and storage bandwidth.

Features of VAAI:

  1. Atomic test & Set (ATS) –  which is used during creation and locking of files on the VMFS volume
  2. Clone Blocks/Full Copy/XCOPY – which is used to copy or migrate data within the same physical array
  3. Zero Blocks/Write Same – which is used to zero-out disk regions
  4. Thin Provisioning in ESXi 5.x and later hosts
  5. Block Delete in ESXi 5.x and later hosts

These features are controlled by the following parameters:

Advanced Parameter name

Description

HardwareAcceleratedLocking

Atomic Test & Set (ATS), which is used during creation of files on the VMFS volume

HardwareAcceleratedMove

Clone Blocks/Full Copy/XCOPY, which is used to copy data

HardwareAcceleratedInit

Zero Blocks/Write Same, which is used to zero-out disk regions

Let’s look at ATS since its relevant to the topic.

ATS is designed to overcome SCSI-2 reservation in which the full volume is locked including metadata. Vmfs allows multiple hosts to access data and to avoid data corruption it effectively handle conflict requests for both data and metadata so file level locking is implemented in SCSI-2 reservation. The SCSI reservation locks the complete LUN and prevent access from other hosts.

So ATS is nothing but SCSI compare-write instead of SCSI-2 reservation. ATS locks only the single sector if it’s a conflicting request instead of blocking the whole LUN.

That being said, now we know why the vmfs5 datastore creation fails, I went ahead and disabled HardwareAcceleratedLocking and was able to get the datastore created. In HP article they have mentioned the issue applies to all P2000’s running in firmware version starting TS230 and TS240. I also knew there was a new revision released by HP last month it starts at TS250. I thought the new firmware might have added this feature in P2000. But in release notes nothing related to VAAI was mentioned. But I anyway updated to the latest. But the issue still exists. I made it work by disabling HardwareAcceleratedLocking

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