Chapter 3
Operation
3-7
The write-through cache mode forces write-through caching to always be used. Read
caching is also performed in this mode.
The none cache mode forces no write caching and no read caching.
Caution –
If there is a failure of one of the redundant components, the cache mode
is set to write-through. In the write-through mode, mirroring is not used and data is
not being cached. If you were to view the cache mode at this time using the
vol
mode
command, the setting for
cache
is displayed as
writethrough
and the
setting for
mirror
is displayed as
on
. This means, however, that mirroring is
enabled, but it is not being used.
The following table highlights the system states that can cause write-behind cache to
be disabled. In cases where write-behind cache has been disabled, the disk tray is
designed to protect the data in cache. If any hardware component fails that might
compromise the safety of this data, the disk tray will disable write-behind cache,
causing outstanding cached data to be flushed to disk. Write-behind caching is
re-enabled when the failed condition has been corrected.
TABLE 2
Cache Transition States
Failure or Problem
Write-Behind Cache State
Disk hard failure (RAID 5)
Disable
Disk hard failure (RAID 1)
Disable
Disk hard failure (RAID 0)
Disable
Disk reconstruct
Disable
Disk copy-back operation
Disable
Interconnect Card 1
Disable
Interconnect Card 2
Disable
Interconnect Card 1+2
Disable (system down)
Controller
Disable (controller failover)
Battery
Disable
Battery warranty expires
Disable
Battery refresh
Disable (hold time low)
14-day battery refresh
Disable (during 12-minute battery discharge)
1
Power and cooling unit 1
Disable
Power and cooling unit 2
Disable
Power and cooling unit 1+2
Disable (system down)
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