Compaq RAID ARRAY 3000 EK-SMCPO-UG. C01 - Manual (Page 44)

2-10 RAID Array 3000 Pedestal Storage Subsystem Hardware User’s Guide
Compaq Confidential – Need to Know Required
Writer: Bob Young Project: RAID Array 3000 Pedestal Storage Subsystem Hardware User’s Guide Comments:
Part Number: EK-SMCPO-UG. C01 File Name: c-ch2 RAID Array Controller.doc Last Saved On: 12/4/00 1:51 PM
Efficient Write and Read Algorithms
Standard RAID write operations that involve parity, such as those in RAID
levels 4 and 5, require multiple, time-consuming steps:
1. Read data from the parity drive.
2. Read existing data from the target data drives.
3. Exclusive-or the old parity, old data, and new data to generate new
parity data.
4. Write the new parity data to the parity drive.
5. Write the new data to the target data drives.
The controller uses several techniques to streamline write operations and
significantly improve performance. All the techniques use the controller’s on-
board cache, which can contain up to 64 MB of memory in the form of
standard 72-pin, 60 ns SIMMs.
NOTE: The controller will not operate without at least one 4 MB SIMM installed in its
cache. Also, it will not operate without either a backup or a UPS connected to the
controller. Without a backup, data stored in the cache, but not yet written to the disk
drives, would be lost in the event of a power interruption.
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 that 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.
Write-back caching makes the host more responsive to the user, since the host
does not have to wait for a lengthy RAID write before proceeding to another
task.