480 Implementing the IBM Storwize V5000 Gen2 with IBM Spectrum Virtualize V8.1
In addition, you can use Real-time Compression along with Easy Tier on the same volumes.
This compression method provides non disruptive conversion between compressed and
decompressed volumes. This conversion provides a uniform user-experience and eliminates
the need for special procedures when dealing with compressed volumes.
9.4.5 Data write flow
When a host sends a write request to the IBM Storwize V5000 Gen2, it reaches the upper
cache layer. The host is immediately sent an acknowledgment of its I/Os.
When the upper cache layer destages to the RACE, the I/Os are sent to the thin-provisioning
layer. They are then sent to RACE, and if necessary, to the original host write or writes. The
metadata that holds the index of the compressed volume is updated if needed, and is
compressed as well.
9.4.6 Data read flow
When a host sends a read request to the IBM Storwize V5000 Gen2 for compressed data, it is
forwarded directly to the Real-time Compression component:
If the Real-time Compression component contains the requested data, IBM Spectrum
Virtualize cache replies to the host with the requested data without having to read the data
from the lower-level cache or disk.
If the Real-time Compression component does not contain the requested data, the
request is forwarded to the Storwize IBM Spectrum Virtualize lower-level cache.
If the lower-level cache contains the requested data, it is sent up the stack and returned to
the host without accessing the storage.
If the lower-level cache does not contain the requested data, it sends a read request to the
storage for the requested data.
9.4.7 Compression of existing data
In addition to compressing data in real time, you can also compress existing data sets
(convert volume to compressed). To do so, you must change the capacity savings settings of
the volume:
1. Right-click a particular volume and select Modify Capacity Settings, as shown in
Figure 9-32 on page 481.
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