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Storage & Clustering Community Blog
Showing posts tagged with Dynamic Multi-Pathing and Dynamic Multi-Pathing for VMware
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Ameya | 03 Aug 2007 | 1 comment
Array Policy Module (APM)

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The APM framework was introduced in Volume Manager 4.0 release. The 4.0 release brought about a major change in DMP architecture - the introduction of APM. As the name suggests, the Array Policy Module (APM) is specific to an array type and defines the policies for an array type. Analogous to its Array Support Library (ASL) counterpart in user space which enables the DDL to identify the array completely, the APM enables DMP kernel to perform array specific operations such as failover, NDU (Non-Disruptive Upgrade), STPG (Set Target Port Groups) and even an I/O policy.

The APM makes it possible for DMP to dynamically add kernel support for an array. The support for enabling an APM is completely online and does not require a reboot. An APM is essentially a dynamically loadable kernel module that is validated and loaded by DMP whenever DMP detects the array type support exported by that APM. In other words, the DDL...

Ameya | 13 Apr 2007 | 1 comment

The first basic function for a multi path (MP) driver is to identify the set of block devices that can be supported. The device discovery is generally triggered when the MP driver is loaded into the operating system (OS) as part of boot process. During early boot, not all devices connected to the system may be available. For example, the devices connected using the fibre channel (FC) cables may not be available during early boot time unless there is special support from the bootstrap, which is usually from NVRAM or EEPROM, to identify the fibre channel devices. In any case, it may be required for a MP driver to rescan the OS device tree at a later stage of boot process.

One of the biggest challenges in device discovery is the ability and skill of MP driver to discover disk devices and their attributes in a non-intrusive fashion such that the disk media is not touched. In other words, the device discovery has to be purely read-only operations to identify the disk and disk...

Ameya | 02 Apr 2007 | 0 comments

The Dynamic Multi Path (DMP) capability was first introduced in Volume Manager 2.5.7 release primarily to support A/A multi path arrays from Sun Microsystems. In those days, the early version of arrays had SCSI connections with fat and thick cables running from host to the array directly. Unlike the present day modern arrays, there was no concept of fabric switches, and no fiber channel (FC) technology enabled on the arrays. The first DMP could do only multi path operations such as load balancing using balanced path I/O policy, path failures and restoration by using SCSI inquiry commands. Since the number of devices were handful, the error processing and restore processing were all single threaded tasks. Further, as the number of supported arrays was small, the entire device discovery and reconfiguration was closely tied to the parent operating system.

The next major enhancements of DMP were introduced in Volume Manager 3.1.1 release that had support to co-exist with...