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RAID Array 3000 Controller Shelf Hardware User’s Guide
Compaq Confidential – Need to Know Required
Writer: Bob Young Project: RAID Array 3000 Controller Shelf Hardware User’s Guide Comments:
Part Number: EK-SMCPQ-UG. D01 File Name: c-ch2 RAID Array 3000 Controller.doc Last Saved On: 12/4/00 1:08 PM
NOTE:
The controller will not operate without at least one 16 MB SIMM installed in its
cache. Nor will it operate without a UPS power supply connected to the controller. Without
a UPS, 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 LUN, 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.
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.
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.