Managing Windows Server 2012 Storage and File Systems : Storage Management (part 3) - Installing and configuring file services - Meeting performance, capacity, and availability requirements

2.2 Configuring multipath I/O

Hardware vendors typically supply a Device Specific Module (DSM) for SAN hardware and software for configuring multipath I/O. That said, the Multipath I/O feature includes the Microsoft DSM and some basic configuration options. The Microsoft DSM supports the Active/Active controller model as well as the asymmetric logical unit access controller model. It also implements path selection policies failover, failback, and load balancing. Failover policies allow you to configure a secondary path that should be used if a preferred path fails. If you want the preferred path to be used automatically when it becomes operational again, you can configure a failback policy.

Several types of load-balancing policies are available, including round-robin, dynamic least queue depth, and weighted path. With round-robin, you can configure the DSM to use all available I/O paths in a balanced, round-robin fashion. With dynamic least queue depth, you can configure the DSM to route I/O to the path with the least number of outstanding requests. With weighted path, you assign each path a weight to indicate its relative priority with regard to a particular application, and the DSM selects the path with the least weight among the available paths.

Devices that support the Active/Active controller model are referred to as Active/Active devices and, by default, are configured to use round-robin. Generally, devices that support the asymmetric logical unit access (ALUA) controller model are compliant with the SCSI Primary Commands-3 (SPC-3) standard or later and, by default, are configured to use failover.

You manage the multipath I/O (MPIO) configuration using the MPIO Properties dialog box, the Mpclaim command-line tool, or the cmdlets of the MPIO module in PowerShell. After you install the Multipath I/O feature using the Add Roles And Features Wizard, these tools are available on the server. You open the MPIO Properties dialog box, shown in Figure 6, by selecting MPIO on the Tools menu in Server Manager.

Note

You can get a list of the available cmdlets for working with MPIO by typing get-command -module mpio at a PowerShell prompt.

Figure 6. Manage the multipath I/O configuration.

After you enable MPIO, you might also want to do the following:

• Enable automatic claiming of iSCSI devices for MPIO.

• Set the default load-balance policy.

• Set the Windows disk timeout.

For MPIO to manage a device, you must first add the hardware ID for the device to MPIO. You can add devices either manually or automatically.

Automatic claiming of iSCSI devices allows MPIO to configure available iSCSI devices with multiple paths automatically. Enable this feature by entering the following at an elevated, PowerShell prompt:

Enable-MSDSMAutomaticClaim -BusType iSCSI

Load balancing and fault tolerance are core features of MPIO. You set the default load-balancing policy using Get-MSDSMGlobalDefaultLoadBalancePolicy. The default policies available are

• Fail over only, which allows one active path, with all other paths designated as standby paths for failover. Use the value FOO.

• Round robin, which sets all available paths to be load-balanced using a round-robin technique. Use the value RR.

• Least queue path, which load-balances by sending I/O to the path with the fewest I/O requests. Use the value LQD.

• Least blocks, which load-balances by sending I/O to the path with the least number of data blocks currently being processed. Use the value LB.

Set the default load-balancing policy by entering the following command at an elevated PowerShell prompt:

Get-MSDSMGlobalDefaultLoadBalancePolicy -Policy PolicyValue

Here, PolicyValue is one of the accepted policy values—either FOO, RR, LQD, LB, or NONE.

You set the timeout value for new disks using Set-MPIOSetting. The basic syntax is

Set-MPIOSetting -NewDiskTimeout NumSeconds

Here, NumSeconds is the number of seconds to wait before reaching the timeout.

Set-MPIOSetting accepts other parameters as well:

• –PathVerifyEnabled When set to –PathVerifyEnabled $true, path verification by MPIO is enabled on all paths according to –PathVerificationPeriod. By default, this feature is disabled.

• –PathVerificationPeriod When –PathVerifyEnabled is set to $true, this parameter sets the interval for path verification. For example, use –PathVerificationPeriod to verify MPIO on all paths every 60 seconds. The default value is every 30 seconds.

• –PDORemovePeriod Controls the amount of time (in seconds) that a multipath pseudo-LUN will remain in system memory, even after losing all paths to the device. When the removal period is exceeded, all pending I/O operations are stopped and set as failed, and the failure is passed on to applications. The default value is 20 seconds.

• –RetryCount Controls the number of times a failed I/O is retried. The default value is 3.

• –RetryInterval Sets the number of seconds to wait before retrying a failed I/O. The default is 1 second.

Before you change MPIO settings, you should determine what the current settings are. You can do this by entering Get-MPIOSetting at the PowerShell prompt.

Adding and removing multipath hardware devices

You manually add devices to MPIO using the MPIO Properties dialog box, which is opened by selecting MPIO on the Tools menu in Server Manager. To manually configure a device to use multipath I/O, follow these steps:

1. Open the MPIO Properties dialog box. On the MPIO Devices tab, you’ll see a list of currently configured multipath devices. If the device you want to work with is not listed, tap or click Add.

2. In the Add MPIO Support dialog box, type the vendor ID as an eight-character string followed by the product ID for the device as a 16-character string. Tap or click OK.

3. You are prompted to restart the server to complete the operation. Tap or click Yes to restart the server.

At an elevated command prompt, you can use Mpclaim to configure devices to use multipath I/O as well. The basic syntax for installing a device follows:

Mpclaim -r -i [-a | -c | -d DeviceId]

The –r parameter indicates that you want to restart the server to allow the device installation to be completed. Although you can suppress the restart using the –n parameter instead of –r, the device will not be installed and available for use until you restart the server. Use the –a parameter to configure multipath I/O support for all compatible devices. Use the –c parameter to configure multipath I/O support for all SPC-3-compliant devices. Use the –d parameter followed by a device’s hardware ID to install a specific hardware device. The hardware ID of a device includes the vendor ID as an eight-character string followed by the product ID for the device as a 16-character string. In the following example, you install a device with EMSVendo0000234767834215 as the hardware ID:

Mpclaim -r -i -d EMSVendo0000234767834215

Alternatively, you can use Get-MSDSMSupportedHw to list available devices by their hardware ID and New-MSDSMSupportedHw to add a device to MPIO.

Using the MPIO Properties dialog box, you can remove a device from MPIO by following these steps:

1. Open the MPIO Properties dialog box. On the MPIO Devices tab, you’ll see a list of currently configured multipath devices.

2. Select the device that should no longer use multiple path IO and then tap or click Remove.

3. You are prompted to restart the server to complete the operation. Tap or click Yes to restart the server.

At an elevated command prompt, you can use Mpclaim to uninstall multipath I/O for a device as well. The basic syntax for installing a device follows:

Mpclaim -r -u [-a | -c | -d DeviceId]

Except for the –u parameter for uninstalling a device, the other parameters are the same as when you are installing MPIO for a device. The following example uninstalls the previously installed device:

Mpclaim -r -u -d EMSVendo0000234767834215

Alternatively, you can use Get-MSDSMSupportedHw to list available devices by their hardware ID and Remove-MSDSMSupportedHw to remove a device from MPIO.

Managing and maintaining MPIO

The MPIO Properties dialog box has several other tabs that you can use for general management of MPIO:

• Discover Multi-Paths When you select the Discover Multi-Paths tab, Windows runs a discovery algorithm to examine added device instances and determine if multiple instances represent the same LUN through different paths. Available multipath devices are then listed by their hardware ID. The hardware ID combines a vendor’s name and a product string that matches a device ID that is maintained by MPIO. Tap or click Add to add hardware IDs for Fibre Channel devices that use Microsoft DSM.

• DSM Install Use the options on this tab to install DSMs provided by independent hardware vendors (IHVs). Keep in mind that many SPC-3-compliant storage arrays can use the Microsoft DSM and you might not need to install an IHV DSM.

• Configure Snapshot Use the options on this tab to save the current MPIO configuration to a log file. Because the log includes details about the DSM, paths, and path states, you can use this information for troubleshooting.

INSIDE OUT: Discover iSCSI and SAS devices

On the Discover Multi-Paths tab, you also can discover iSCSI devices and SAS devices and add MPIO support for the discovered devices automatically. To discover iSCSI devices, select the Add Support For iSCSI Devices check box. To discover SAS devices, select the Add Support For SAS Devices check box. Adding support for iSCSI, SAS, or both allows the Microsoft Device Specific Module (MSDSM) to claim all iSCSI, SAS, or both devices for MPIO.

You configure the load-balancing policy for LUNs using their disk properties. In Computer Management, select Disk Management and then press and hold or right-click the disk you want to work with. In the Properties dialog box, click on the MPIO tab. Use the Select MPIO Policy list to choose the load-balancing policy for the selected disk. If you use Failover Only as the load-balancing policy, you can configure a preferred path to the storage. This path is used for automatic failback.

Whether you are working with internal or external disks, you should follow the same basic principles to help ensure that the chosen storage solutions meet your performance, capacity, and availability requirements. Storage performance is primarily a factor of the disk’s access time (how long it takes to register a request and scan the disk), seek time (how long it takes to find the requested data), and transfer rate (how long it takes to read and write data). Storage capacity relates to how much information you can store on a volume or logical disk.

Although early NTFS implementations limited the maximum volume size and file size limit to 32 GBs, later implementations extended these limits. This means you can have a maximum NTFS volume size of 256 TBs minus 64 KBs when you are using 64-KB clusters, and 16 TBs minus 4 KBs when you are using 4-KB clusters. The maximum file size on an NTFS volume is 16 TBs minus 64 KBs. Further, a maximum of 4,294,967,294 files can be created on each volume, and a single server can manage hundreds of volumes (theoretically, around 2000).

Storage availability relates to fault tolerance. You ensure availability for essential applications and services by using availability technologies. If a server has a problem or a particular application or service fails, you have a way to continue operations by failing over to another server. In addition to clusters, you can help ensure availability by saving redundant copies of data, keeping spare parts, and if possible making standby servers available. At the disk and data levels, availability is enhanced by using redundant array of independent disks (RAID) technologies. RAID allows you to combine disks and to improve fault tolerance.

RAID can be implemented in hardware or software. When servers have hardware RAID controllers installed, the internal controller can be used to implement RAID on the server’s internal disks. When a server is allocated storage from a storage array, one or more logical unit numbers, or LUNs, are assigned. Each LUN is a virtual disk. Typically, hardware RAID configured within the storage array is used to spread the LUN across multiple physical disks (also called spindles).

Windows Server 2012 supports several software RAID options, including traditional software-based RAID and Storage Spaces. Traditional software RAID is the software-based RAID technology built into the operating system and available in earlier releases of Windows. Storage Spaces provide resilient storage using new technologies and are preferred over traditional software RAID. However, each of these software-implemented RAID levels requires processing power and memory resources to maintain. By using hardware RAID, you use separate hardware controllers (RAID controllers) to maintain the disk arrays. Although this requires the purchase of additional hardware, it takes the burden off the server and can improve performance. Why? In a hardware-implemented RAID system, a server’s processing power and memory aren’t used to maintain the disk arrays. Instead, the hardware RAID controller (which is installed internally or in a storage array) handles all the necessary processing tasks.

The RAID levels available with a hardware implementation depend on the hardware controller/storage array and the vendor’s implementation of RAID technologies. Some hardware RAID configurations include RAID 0 (disk striping), RAID 1 (disk mirroring), RAID 0+1 (disk striping with mirroring), RAID 5 (disk striping with parity), and RAID 5+1 (disk striping with parity plus mirroring). Table 1 provides a summary of these RAID technologies. The table entries are organized listing the highest RAID level to the lowest.