Compaq RAID ARRAY 3000 EK-SMCPO-UG. C01 - Manual (Page 45)

RAID Array Controller 2-11
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
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 action enables the controller to avoid reading from the parity and data
drives before making the write. All the controller 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.
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.