MOLAR: A cost-efficient, high-performance hybrid storage cache

Hybrid system analysis and design for scale-out storage environments

“…In this type of storage architectures [42,43,53,66], when a new write request arrives and its accessing data is not located in SSD, the data needs to be recorded to HDD and the request is completed only when the recording is successful. If the access data is available in SSD, the data in HDD needs to be updated, and the data in SSD may be discarded, or updated as well.…”

“…To reduce the unnecessary write operations, [42] proposes a method to check the data hotness based on the demotion counter and the proposed control metric, and migrate the hot data blocks to SSD. In [66], a heuristic file-placement algorithm is designed to improve the cache performance by considering the IO patterns of the incoming workloads.…”

“…In [17], a hybrid structure that combines SSD and SMR devices is proposed, which utilizes SSD as the read-only cache to improve the random read performance. In [42], the number of demotions from DRAM to SSD is recorded and utilized to predict the future accesses to SSD, and decide whether the data need to be recorded in SSD or not.…”

“…In [44], the lazy adaptive cache replacement algorithm can also reduce the chance of copying cold data to SSD, which also improves the read cache hit ratio and the overall system performance. In [42], the demotion operations are counted as the reference to decide the data movement between SSD and HDD, which has similar effects as setting the data hotness count utilized during the hot data identification algorithm. To improve the data replacement policy for the write cache, [46,47] split the cache space into read and write regions, and read region only handles the hot read data, while write region handles the write requests.…”

“…To take care of the issue of limited SSD write cycles, researchers have proposed lots of storage algorithms [39,44,53,74] and architectures [48] to avoid the unnecessary writes to SSD. Some of them [42,43,53,66] apply SSD as the read-only cache to save the SSD lifetime, however, it decreases the write performance. Some researches [107,108] even treat HDD as the write cache for SSD to save the SSD lifetime at the cost of reducing overall system performance.…”