IBM System Storage DS4000, Hardware Manual

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Chapter 3. IBM System Storage DS4000 and DS5000 hardware
39
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.
Figure 3-12 Simplified data cache flow diagram
Write
DMA write
to Cache
Yes
Yes
No
Yes
No
Yes
No
No
Yes
No
Read
No
No
Write Through
Yes
Yes
I/O Request
(Controller A)
Yes
Read
or
Write?
Is data
in cache?
Read data
from Cache
(cache hit)
Write
caching
enabled?
Set cache
block
Dirty WT
Battery
OK?
Caching
w/o
battery?
Read data
from disk
(cache miss)
Read/V next
blocks to
cache
Sequential
read?
I/O
complete
Set cache
block
Dirty WB
Mirror
Cache?
Set cache
block
Dirty WBM
Copy to
Controller B
Wait for
confirmation
Remote
Mirror?
Write-Back
Wait for confirmation
in Metro Mirror
Remote
mirror
operation
WB or
WT
mode?
I/O
complete
Write data to disk
Set cache block
clean