IBM Midrange System DS4000 Series Hardware Manual page 57

Figure 3-12 shows a simplified cache data flow for a DS storage subsystem. It does not cover
remote mirror operations and parity calculations for RAID 3, 5, or 6. In the example, I/O size is
equal or smaller than segment size.
DMA write
to Cache
Write
caching
enabled?
Yes
Caching
w/o
battery?
Yes
Set cache
block
Dirty WB
Yes
Remote
mirror
operation
Wait for confirmation
in Metro Mirror
Figure 3-12 Simplified data cache flow diagram
I/O Request
(Controller A)
Read
Write or Read
Write?
Set cache
No block
Dirty WT
No
Battery
No
OK?
Yes
Set cache
Mirror
Yes
block
Cache?
Dirty WBM
Copy to
No
Controller B
Wait for
confirmation
Remote
Mirror?
No
WB or
Write Through
WT
mode?
Write data to disk
Set cache block
Write-Back
clean
I/O
complete
Chapter 3. IBM System Storage DS4000 and DS5000 hardware
Read data
Is data
Yes from Cache
in cache?
(cache hit)
No
Read data
from disk
(cache miss)
Read/V next
blocks to
cache
Sequential
Yes
read?
Yes
No
I/O
complete
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