Abstract
Solid-State Drives (SSDs) offer significant performance
improvements over hard disk drives (HDD) on a number
of workloads. The frequency of garbage collection (GC) activity
is directly correlated with the pattern, frequency, and volume
of write requests, and scheduling of GC is controlled by logic
internal to the SSD. SSDs can exhibit significant performance
degradations when garbage collection (GC) conflicts with an
ongoing I/O request stream. When using SSDs in a RAID array,
the lack of coordination of the local GC processes amplifies
these performance degradations. No RAID controller or SSD
available today has the technology to overcome this limitation.
This paper presents Harmonia, a Global Garbage Collection
(GGC) mechanism to improve response times and reduce performance
variability for a RAID array of SSDs. Our proposal
includes a high-level design of SSD-aware RAID controller and
GGC-capable SSD devices, as well as algorithms to coordinate
the global GC cycles. Our simulations show that this design
improves response time and reduces performance variability
for a wide variety of enterprise workloads. For bursty, write
dominant workloads response time was improved by 69% while
performance variability was reduced by 71%.
