IBM System Storage DS4000, Hardware Manual

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IBM Midrange System Storage Hardware Guide
Storage subsystem cache flushing levels
These two settings affect the way the cache controller handles unwritten cache entries. They
are only effective when the write-back cache policy is configured. Writing the unwritten cache
entries to the disk drives is called
flushing
. The start and stop flushing level values can be
configured, as shown in Figure 4-114. They are expressed as percentages of the entire cache
capacity. When the number of unwritten cache entries reaches the start flushing value, the
controller begins to flush the cache (write the entries to the disk drives). The flushing stops
when the number of unwritten entries drops below the stop flush value. The controller always
flushes the oldest cache entries first. Unwritten cache entries older than 20 seconds are
flushed automatically.
Figure 4-114 Storage subsystem cache settings
The default is the start flushing level and the stop flushing level is set to 80%. This means that
the cache controller does not allow more than 80% of the entire cache size for write-back
cache, but it also tries to keep as much of it as possible for this purpose. If such settings are
used, a high number of unwritten entries in the cache is expected. This is good for writing
performance, but be aware that it offers less data protection.
Performance tests have shown that it is a good idea to use similar values for start and stop
flushing levels. If the stop level value is significantly lower than the start value, this causes a
high amount of disk traffic when flushing the cache. If the values are similar, then the
controller only flushes the amount needed to stay within limits.
Note: In our experience, a start flushing level of 50% and a stop flushing level of 50% is
best for customer environments.