IBM Midrange System DS4000 Series Hardware Manual page 56

5. The cache block status is set to Clean.
6. The host receives the message I/O completed (if it did not receive this message before).
Read operation steps
The read operation steps are:
1. A host requests data.
2. If data is in the cache, the data is sent to the host and the I/O is completed.
3. Data is read from disk.
4. Data is copied into cache.
5. The host receives the required data and the I/O is completed.
6. If sequential reading is discovered, the next n segments are read and copied into the
cache.
Cache block states
The cache block states are:
Empty: During the start-of-day processing, the controller firmware will partition the cache
memory into cache blocks (except for the part of cache memory reserved for various
cache metadata). All these cache blocks will be put on the Free list and their state will be
Empty.
Clean: A cache block that contains data that is consistent with the data on disk is Clean.
Dirty Write-Through (Dirty WT): A cache block that contains new data that is not
consistent with data on disk is Dirty. In the write-through cache mode, status is not
returned to the host until the data has been written to disk. The cache block holding the
write data is Dirty Write-Through until the data is written to disk and the cache block
becomes Clean.
Dirty Write-Back (Dirty WB): In the write-back cache mode, the status is returned to the
host as soon as the data has been inserted into the cache. The cache block holding the
write data is Dirty Write-Back until the data is written to disk and the cache block becomes
Clean.
Dirty Write-Back Mirrored (Dirty WBM): If cache mirroring is enabled for the volume, the
dirty cache block will be replicated to the cache memory in the other controller, and when
the replication is completed, the cache block transitions from Dirty Write-Back to Dirty
Write-Back Mirrored.
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IBM Midrange System Storage Hardware Guide