Increasing The Amount Of Raid 0/1 Space Available; Why Deleting Files Won't Increase Raid 0/1 Space - HP Surestore Disk Array 12h - And FC60 Service Manual

Concepts and Management
Optimizing Performance

Increasing the Amount of RAID 0/1 Space Available

CAUTION! Deleting a logical drive (LUN) destroys all data on the logical drive. Before
deleting a logical drive, make backup copies of the files you want to save or move the files
to another logical drive.
If the write working set is exceeding the amount of available RAID 0/1 space, you can restore performance
by increasing the amount of RAID 0/1 space. You can do this in one of the following ways:
Add a disk and leave its capacity unallocated. This is an effective way of permanently increasing the
amount of RAID 0/1 space available for the write working set.
Delete an unneeded existing logical drive and leave its capacity unallocated. This too will permanently
increase the amount of RAID 0/1 space available for the write working set. Move the data to another
device.
Add a disk and create a new logical drive with its capacity. This is a temporary way of increasing the
amount of available RAID 0/1 space. As the new logical drive begins to fill up with data, it will be
converted to RAID 5 space and you may again find that the available 10% RAID 0/1 minimum is too
small to accommodate the write working set.
Delete a logical drive and recreate it. This has the same temporary effect as adding a new disk and
creating a logical drive.

Why Deleting Files Won't Increase RAID 0/1 Space

It might appear that simply deleting some unneeded or archived files from the disk array would increase the
amount of RAID 0/1 space available for the write working set. However, this is not the case due to the way
the disk array (and most other disk systems) handles deleted files.
When you delete files from the disk array, the operating system updates its file system to record the
deletion. From the operating system's perspective there is now more disk capacity available. However, the
disk array itself is unaware of the deletion and keeps the data files intact. From the disk array's perspective,
the amount of stored data has not changed. Consequently, a disk array that has reached the 10% RAID 0/1
minimum cannot increase the RAID 0/1 space when files are deleted because none of the data stored in
RAID 5 has been deleted.
This situation represents a trade-off between disk array performance and operating system file
management. Most operating systems have the ability to recover ("undelete," or "salvage") files that have
been deleted. They can do this because the data remains on the disk media. If the disk array really "deleted"
files to regain RAID 0/1 space, you would no longer be able to use these operating system recovery
features.
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