SSD Cache
193
10.2.6.
SSD Cache Tuning
The SSD cache can be tuned to maximize its efficiency base on application usage. Cache
block size, populate-on-read threshold and populate-on-write-threshold are the main
parameters.
Cache Block Size
A large cache block suits applications where frequently accessed data is close to each
other, known as a high locality of reference. A large cache block will also fill up the SSD
cache quickly - this is known as the warm-up time. After the cache is warmed up, the
performance would be quite good for applications with high locality of reference. Such as
the file system or web service usage, the frequently accessed data are based on some
concentrated files which are usually in large block size. However large cache blocks will
also generate larger I/O overhead, increasing response time, especially for cache misses.
A smaller cache block size suits applications with data that is less localized, meaning the
data is accessed more randomly, such as database usage. The SSD cache will fill up slower,
but with more cache blocks, there is greater chance of a cache hit, especially for data with
less locality of reference. With a smaller cache block size, cache usage is usually less than
with a larger cache block size, but overhead is less, so the penalty for cache misses is less
severe.
Population Threshold
The population threshold is the quantity of accesses at which point that cache block is
copied to the SSD Cache. A higher number ensures that the cache only stores frequently
accessed data so there will not be much cache turnover however it also means the cache
will take longer to warm up and be fully effective. A lower number means the cache is
warmed up quickly, but may cause excessive cache populations. A populate on read
threshold of 2 is sufficient for many applications. Populate-on-write is useful when data that
is written to is often read soon after. This is often the case in file systems. Other
applications, such as database software, does not have this tendency so populate on write
may sometimes even be disabled.
Table 10-3
I/O Type Table for SSD Read Cache
I/O Type
Block Size
(Sectors)
Populate-on-Read
Threshold
Populate-on-Write
Threshold
Database
1MB (2,048)
2
0
File System
2MB (4,096)
2
2
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