Hitachi Dynamic Link Manager (HDLM)

Hitachi Dynamic Link Manager (HDLM)

What is the HDLM: it is a server –based software solution that directly addresses challenges associated with the single point of failure.

iSCSI devices , Hitachi storage system command devices, such as Hitachi RAID Manager command devices, EMC DMX series, EMC CX series, and HP EVA series are managed by HDLM and tape devices and the disks on the host devices are not managed by HDLM.

HDLM features.

1.     Multipathing: Multiple paths can also be used to share I/O workloads and improves performance.

2.     Path failover: By removing the threat of I/O bottlenecks, HDLM protect your data paths and increases performance and reliability.

3.     Failback: By recovering a failed path and placing it back online when it becomes available, the maximum number of paths available for load balancing and failover is assured.

4.     Load Balancing: By allocating I/O request across all paths, load balancing ensures continuous operations at optimum performances levels, along with improved system and application performance. Several load balancing policies are supported.

Since HDLM automatically perform path health checking, the need to perform repeated manual path status checks is eliminated.

 

With multi-pathing, a failure with one or more components still allows applications to access their data. In addition to providing fault tolerance, multi-pathing also serves to redistribute the read/write load among multiple paths between the server and storage, helping to remove bottlenecks and balance workloads. In addition, distributing data access across all the available paths increases performance, allowing more application to be run and more work to be performed in a shorter period of time.

 

How HDLM works:

1.     HDLM driver interfaces with HBA driver or multipathing framework provided by OS.

2.     Assign a unique identifier to paths between each storage devices and host.

3.     Distributes application I/O across each path according to fail-over and load balancing.

4.     When a path fails, all outstanding and subsequent I/O request shift automatically and transparently from failed or down path to alternative paths.

 

Two types of failover happen: Automatic and manual.

 

Failover keeps your mission critical operation running without interruptions, storage assets is maximized, and business operations remain online.

 

The path can go offline due to the following reasons.

1.     An error occurred on the path.

2.     A user intentionally placed the path offline by using the path management window in the HDLM GUI.

3.     A user executed the HDLM commands offline operations.

4.     Hardware, such as cables or HBAs, has been removed.

 

You can manually place a path online or offline by doing the following:

1.     Use the HDLM GUI path Management window.

2.     Execute the “dlnkmgr” command’s online or offline operation.

The algorithms used for the load balancing are the “round robin”. This algorithm simply distributes I/O by alternating request across all available data paths. Some multipath solutions, such as the IBM MPIO default PCM, only provide this type of load balancing.

If we use the Extended Round-robin for the load balancing then it distributes I/O to paths depending on whether the I/O involves sequential or random access:

. For sequential access, a certain number of I/O is issued to one path in succession. The next path is chosen according to the round robin algorithm.

. For random access, I/O is distributed to multiple paths according to the round-robin algorithm.

To centrally manage the multiple HDLM instances by Hitachi Global Link manager (HGLM).

By HGLM we can centrally administrator multiple HDLM multipath environment from single point of control and consolidate and present complex multipath configuration information on simplified host and storage centric views.

 

Summary:

1.     Provides a centralized facility for managing path failover, automatic failback, and selection of I/O balancing techniques thorugh integration with Hitachi Global link Manager.

2.     Eases installation and use through auto-discovery function, which automatically detects all available paths for failover and load balancing.

3.     Provides one path-management tool for all operating systems, Includes the CLI that allows administrators flexibility in managing paths across networks.

4.     Provides manual and automatic failover and failback support.

5.     Monitors status of online paths through a health –check facility at customer-specified intervals, and places a failed path offline when an error is detected.