RAID Array 3000 Controller Shelf
2.5.2.1 Write-Back Caching
When the host sends data to be written to a redundancy group the controller stores the data in its cache and immediately reports to the host it has completed the write. The controller eventually writes the data to the disk drives when the write can be done most efficiently, or when the controller must flush the cache to make room for other data or to prepare for a shutdown.
2.5.2.2 Write Gathering
The controller will attempt to consolidate multiple writes destined for contiguous blocks and then write the entire data block in one operation. The controller stores the data in cache until it performs the write. Ideally, the controller will wait until it has gathered enough data to fill an entire stripe. This enables the controller to avoid reading from the parity and data drives before making the write. All it has to do is calculate parity from the data it already has in its cache, then write the data and parity to the drives. Even if the controller cannot accumulate enough data to fill a stripe, the consolidation of small writes can reduce the number of read/write operations that must take place.
2.5.2.3 Write On Top
If the host commands that data be written to disk, and data for that address is pending in the controller’s cache, the controller writes the new data on top of the old in the cache. Only the new data is eventually written to the disk drives.
2.6RAID Levels Supported
The RAID Array 3000 controller supports the following RAID levels:
Table 2–2 RAID Levels Supported
RAID Level
0
1
0+1
4
5
JBOD
Description
Striping without parity
Mirroring
Striping and mirroring
Striping with fixed parity drive
Striping with floating parity drive
“Just a Bunch of Drives”
