EMC Symmetrix DMX-3, Руководство по эксплуатации продукта

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112 EMC Symmetrix DMX-3 Product Guide
Symmetrix DMX-3 Input/Output Operations
Write destaging
operation In addition to the four types of I/O operations explained earlier,
Symmetrix systems perform a background operation that destages
blocks back to disk. This allows any written-to or changed data to be
maintained in two locations: in global memory for performance in the
occurrence of re-use of that data and on disk to maintain the highest
levels of data integrity. Any pending writes are assured of arrival to
the intended disk, even in the event of power failure.
Figure 30 on
page 112 illustrates this destaging operation.
Figure 30
Destaging operation
For each logical volume the Symmetrix system maintains a special
data structure that points to the data that needs to be destaged in the
global memory Write Pending Indicators (WPIs) already discussed
above. This dynamically adjusting algorithm saves disk seek and
latency time by destaging data in groups of up to four tracks
concurrently per logical volume.
Channel
Channel
director
Directory
Global
memory
Disk
directo
r
Disk
1 Destage blocks
2 Update directory