Abstract
NAND flash memory is a preferred storage media
for various platforms ranging from embedded systems to
enterprise-scale systems. Flash devices do not have any mechanical
moving parts and provide low-latency access. They
also require less power compared to rotating media. Unlike
hard disks, flash devices use out-of-update operations and they
require a garbage collection (GC) process to reclaim invalid
pages to create free blocks. This GC process is a major cause
of performance degradation when running concurrently with
other I/O operations as internal bandwidth is consumed to
reclaim these invalid pages. The invocation of the GC process
is generally governed by a low watermark on free blocks and
other internal device metrics that different workloads meet
at different intervals. This results in I/O performance that is
highly dependent on workload characteristics. In this paper,
we examine the GC process and propose a semi-preemptive
GC scheme that can preempt on-going GC processing and
service pending I/O requests in the queue. Moreover, we further
enhance flash performance by pipelining internal GC operations
and merge them with pending I/O requests whenever possible.
Our experimental evaluation of this semi-preemptive GC sheme
with realistic workloads demonstrate both improved performance
and reduced performance variability. Write-dominant workloads
show up to a 66.56% improvement in average response time with
a 83.30% reduced variance in response time compared to the
non-preemptive GC scheme.
