Technical Report MSC-2017-18

Title: Improving SSD-based Caches Lifetime with Write-Once Memory Codes
Authors: Ran Koretzki
Supervisors: Eitan Yaakobi, Assaf Shuster
Abstract: Solid State Disks (SSDs) have the potential to revolutionize the storage system landscape. They have gained popularity as cache devices in data centers because, they are faster in read and write operations and have lower power consumption, compared to the traditional magnetic hard disks (HDD). However, SSDs have a limited number of times it can write to a single physical location, and there is a second limitation. The SSD must perform an erase operation before it can write to the same location again. Write-once memory (WOM) codes were studied as a way to increase the number of writes on a write-once storage medium to more than once. In previous studies, the usage of WOM-codes in SSD was studied to increase its lifetime by 50%; theoretically, reusing invalid pages for additional writes, and thus reducing the number of erase operations. The additional writes are inflated when using WOM-codes and come with the cost of utilizing the overprovisioning space in SSDs to maintain constant external capacity. In cache devices, there is no such constraint on external capacity, so we could use a more significant percentage for second writes in order to extend a bit further the SSD lifetime. In this work, we propose a caching algorithm for SSD-based caches that will leverage WOM-codes to reuse invalid pages for gaining an additional number of writes in SSD. We study the use of these additional writes to improve in both cache hit ratio and SSD lifetime, without the constraint of maintaining constant external capacity.
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