Points to remember when expanding the RAID group in Hitachi HUS (Hitachi Unified Storage)

Points to remember when expanding the RAID group in Hitachi HUS (Hitachi Unified Storage)

Raid Group can be expanded on Hitachi HUS storages, but we need to take lot of care during the raid group expansion, if you will not take care of some of points during expansion then you will end up with data loss.

1.       Raid Group can be expanded by adding the disk.

2.       Minimum disks that can be added are 8 or less if we reach the maximum RG width.

3.       R0 cannot be expanded.

4.       Any number of RG expansion request can be given but at any point of time only each controller will do one raid group expansion.

5.       Expanding the raid group will not expand the luns inside the raid group luns are known as LU in Hitachi.

6.       When the Raid group is expanded then extra space get created in which we can create extra LUs in that raid group.

7.       Raid group expansion takes times and slightly performance gets decrease so it is recommended by Hitachi to do this operation when the I/Ops are less on Hitachi storage.

8.       Expanding the raid group will not change the raid level means the R5 (raid 5) will be R5 (raid5) when it get expanded.

9.       Only those RG can be expanded whose PG depth is 1.

10.   Raid group can be expanded but cannot be shrunk.

11.   There are two states when the raid group is expanded, one is the waiting state and one is the expanding state.

12.   Waiting state raid group state can be cancelled, in this state means that the raid group expansion is yet not started and then you can cancel it if you want.

13.   Expanding state raid group cannot be cancelled, means the expansion got started and if we forcefully try to cancel it then we can end up by data loss.

Rules for expanding a RAID group

You cannot expand the raid group in following conditions

1.       If the LU whose status of the forced parity correction is:

Correcting, waiting, waiting drive reconstruction, unexecuted, unexecuted 1 or unexecuted 2.

Means that when parity construction is going on please don’t perform the raid group expansion, let the activity get completed and then we can do the expansion.

2.       If an LU is being formatted and its part of the raid group which you need to expand then don’t expanded the raid group until and unless the formatting gets completed.

3.       If you are expanding a raid group after setting or changing cache partition manager configuration, the storage system must be rebooted, expand the raid group after rebooting the storage system in which the power saving function is set. Change the status of the power saving features to “Normal (spin-on)” and then expand the raid group.

4.       If you are expanding a raid group when the dynamic sparing/correction copy/copy back is operating, expand the raid group after the drive has been restored.

5.       If you are expanding a raid group while installing the firmware, expand the raid group after completing the firmware installation.

Best practices for raid group expansion

1.       You can assign priority as Host I/O or RAID group expansion.

2.       Perform backup of all data before executing expansion.

3.       Execute the RAID group expansion at a time when host i/o is at a minimum.

4.       Add drive with the same capacity and rotational speed as RAID group of expansion target to maximize performance.

5.       Add drives in multiples of 2 when expanding a RAID-1 or RAID-1+0 groups.

Now in 2 point it is mentioned to take backup of all the data before doing the expansion is because some time during expansion , if there is power failure, or something happens to the system and if any disaster happens, then the LUN associated with the expansion can become unformatted. And there can be chance of data loss. So to be at safer side please takes a backup of the data before performing the expansion.