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Friday, November 29, 2013
Hitachi Combines SSD Tier And SSD Cache Methods in Tiered
Hitachi has evaluated the methods currently followed to utilize SSD on the storage having both SSDs and HDDs, and proposed a new one that increases the storage I/O performance.
The amount of access workload to information systems has been
increasing. In general, input and output (I/O) activities have
a locality, and the number of I/Os for each area is different.
Typically, frequently accessed areas are stored on high
performance but still expensive SSDs (Solid State Drives), and
rarely accessed areas are stored on low performance and more
affordable HDDs (Hard Disk Drives). The result is a increased
I/O performance and a reduced system cost.
There are two methods to utilize SSD on the storage having both
SSD and HDD.
- SSD Tier Method: Method Dividing data and locating the data
to SSD or HDD
- SSD Cache Method: Method locating all data to HDD and coping
partial data to SSD
The SSD tier method improves I/O performance by allocating hot
areas to the SSD. However, it takes 1 hour to 24 hours to
improve the I/O performance because this method optimizes data
location every 1 hour to 24 hours when the hot areas move.
Since the SSD cache method caches accessed area to SSD
immediately, it improves the second and subsequence accesses to
the area. Performance does not improve when cached data is
frequently purged form SSD cache before data is accessed.
During the Second Asian Conference on Information Systems (ACIS
2013) held earlier this month in Phuket, Thailand, Yokohama
Research Laboratory (YRL), Hitachi Ltd. presented a paper
titled "Evaluation of SSD Tier Method and SSD Cache Method in
Tiered Storage System."
Hitachi's researchers considered that by utilizing both methods
simultaneously addresses the problems mentioned above. Assuming
that the appropriate number of SSDs for tier and appropriate
number of SSDs for cache may differ on the basis of the number
of I/O, read-write ratio, and I/O locality, the researchers
evaluated the effects of these methods by I/O simulation and
clarified the conditions under these methods are effective.
The simulation result showed that the SSD tier method improves
the storage I/O performance in write-intensive and low I/O
locality workload with small SSD capacity. The SSD cache method
is effective in read-intensive and high I/O locality workload
with large SSD capacity. Hitachi claims that utilizing both
methods simultaneously with write-intensive workload is the
"The storage I/O response time is reduced by up to 28%
utilizing both methods simultaneously compared to using the SSD
tier method or the SSD cache method separately," the