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Qsan Document – User Manual
The following steps describe details about a host read with a cache hit:
1.
A host requests a read data.
2.
Read data from the SSD.
3.
Return requested data to the host.
4.
If SSD has error, read data from the HDD.
Write Data
The following figure shows the steps of the controller which handles a host write request.
The following steps describe details about a host write:
1.
A host requests a write data.
2.
Write data to the HDD.
3.
Return the status to the host.
4.
Populate the cache to SSD.
I/O Type
The type of I/O access is a user-selectable SSD cache configuration. The user-selectable I/O type
controls the SSD cache internal settings for cache block size, sub-block size, populate-on-read
threshold, and populate-on-write threshold. Three pre-defined I/O types are supported; they are
database, file system, and web service. The user can select an I/O type to set the SSD cache of a
virtual disk. When enabled SSD caching, the user can also change it online. But the cached data
would be purged if the I/O type is changed. You may select the suitable I/O types depends on the
HDD
SSD
Memory Cache
Data
2. Write to HDD
4. Populate Cache
1. Request Write
3. Return Status
