Adding a Full Hard Disk Drive to an Array Version 1.0
Purpose: This document explains the issues involved when adding a completely full hard drive to a RAID while attempting to preserve all the data on the drive.
When using RAID1 (Mirroring), it is possible to add a hard drive that already contains data to an array and have the data automatically mirrored to another drive in the array without having to manually copy the data. In order to add the drive to an array, the RAID software creates a reserved sector on the hard disk. If this sector has data on it, that data will be lost.
How Promise adds a Hard Drive to an Array
When the User specifies a hard disk drive to be part of an array, the Promise RAID BIOS reserves a 32 KB sector on the last (innermost) cylinder of the hard drive. This sector contains information that the RAID engine uses to identify the hard drive, read data from and write data to it accordingly.
The last cylinder of the hard drive was chosen as the location for the reserved sector because it is theoretically the last place on the drive that data would be recorded, thus it poses the least amount of interference with reading from and writing to the drive.
With RAID1, the data from one hard drive is mirrored to another drive in the array. Thus it is possible to add a hard drive that already contains data to an array and have the data automatically mirrored to another drive without having to manually copy the data.
Adding a Full Hard Drive to an Array
As mentioned previously, in order to add the drive to an array, the RAID software must create a 32 KB reserved sector on the last cylinder of the hard disk. It is unlikely that the disk will be so full of data that this sector will already have data on it. However, if there is data in this sector, that data will be lost when the drive is added to an array.
When a completely full drive must be added to a RAID array the following actions will reduce or eliminate data loss from a reserved sector overwrite.
- If the hard drive comes from a personal computer or workstation, it may have a GB or more of unused temporary files. Run a disk cleanup utility to find and delete these files.
- Run a defragmenter utility. This will move all the free space to the last cylinders of the disk, virtually eliminating the possibility of an overwrite.
If the defragmenter utility cannot function because there is inadequate scratch space, temporarily move some of the data, ten percent, for example, off the hard drive to provide the needed space. After defragmenting the hard drive, move the data back.
Prevention is Better than Problem Solving
There are several well-known steps to minimize the probability of the above situation happening.
- Monitor the hard drive periodically.
- Impose a limit to the amount of data you save to a hard drive, such as such as 95 percent of its total capacity.
- Use a file compression utility to save space on the hard drive.
- Use Disk Cleanup to remove unneeded files.
- Run a defragmenter utility to eliminate fragmentation and improve hard drive performance.
About the Author
Syed Yasir Hashmi http://www.yasir.in is an I.T guru working in the industry for the last 12 years
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