If the reference is to the front of a partial track image, stage the track from the
point of reference up to the start of the partial track image and treat the
operation as a hit. If the reference is to HA or R0 of the track, stage the track
from index to the end of data. Execute the channel program using the cache
image. If the reference is to the middle of the track, stage the track from the
point of reference to end of data. Execute the channel program using the
cache image. Write the data into the cache and write it to the disk subsystem.
MRU the track image in the cache.
Format Write Operation - Search on ID
Perform the search using the Track Format Descriptor Table. Receive the data
from the channel into the cache and write it to the disk subsystem. MRU the
track image in the cache.
Unpredictable Track Format
Only those 16 KB sections of unpredictable tracks which have been formatted are
staged up.
Read Operation
Stage the track from index to the end of data. Execute the channel program
using the cache image and MRU the track at the end of the operation.
Write Operation
Stage the track from index to the end of data. Execute the channel program
using the cache image. Do not keep a cache image of the track.
Note:
In the rare event that the track format is unknown, stage all blocks possibly
associated with the logical track and build the data for the track information
table. Treat the track as in the hit case.
Algorithms for Cache HITS
Read Operation
Execute the channel program using the cache image and MRU the track at the
end of the operation.
Write Operation
Execute the channel program using the cache image and MRU the track at the
end of the operation. Write the updated sectors to the disk subsystem.
Algorithm Modifications due to Cache Hints
Bypass Cache
– Read Miss
Read only the requested data into the cache and execute the channel
program as in normal cache replacement but do not keep a copy of the
track in the cache.
– Read Hit
Even though there is a current copy of the data, read the data into the
cache and execute the channel program using this cache image. Discard
this cache image.
B-2
Internal Disk Subsystem Reference Guide
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