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.
