SSD Cache
183
10.2.
Theory of Operation
SSD cache allows an SSD to function as read cache or write buffer for a HDD volume. In
SSD read cache, it is a secondary cache that improves performance by keeping frequently
accessed data on SSDs where they are read far more quickly than from the HDD volume.
When reads or writes are performed, the data from the HDDs are copied into the SSD cache.
Although the data is duplicated to SSD cache pool, it does not matter if the read cache pool
is corrupted.
In SSD write cache, SSDs are a write buffering storage that improves performance by
storing the write data in SSDs temporary where they are write far more quickly than to the
HDD volume. And then the write data will be flushed to the HDD volume at the appropriate
time. It may take risk of losing data during the period that the write data is stored in SSD
cache if the SSD cache pool is corrupted. The write data has not yet written back to the HDD
volume. So the read-write cache pool needs data protection to protect the write data.
CAUTION:
Using SSD read-write cache, it may take risk of losing data if the SSD
cache pool is corrupted. User has to monitor the health of the SSD cache
pool carefully.
10.2.1.
System Memory and SSD Cache Capacity
SSD cache function needs system memory as index. The usable capacity of SSD cache is in
proportion to the size of the controller system memory. The following table is the
relationship between system memory and SSD cache capacity.
Table 10-1
The Relationship between System Memory and SSD Cache Capacity
System Memory per Controller
Maximum SSD Cache Capacity per System
4GB
X (Not Support)
8GB ~ 15GB
2TB
16GB ~ 31GB
4TB
32GB ~ 63GB
8TB
64GB ~ 127GB
16TB
128GB
32TB
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