Quality of Service Implications of the Error Correction Techniques in Solid State Drives

Lorenzo Zuolo†, Cristian Zambelli†, Rino Micheloni‡, Stephen Bates‡ and Piero Olivo† †Dipartimento di Ingegneria – Università di Ferrara (Italy)

‡PMC-Sierra lorenzo.zuolo@unife.it

Solid State Drives (SSDs) are now the most effective solution for mass storage applications

• High performance• Read and write bandwidth up to GB/s• Million IOPS

• High robustness• No mechanical parts

• Low power consumption• Power wall at 25W

Reliability?SSD’s reliability is tightly coupled with that of the exploited storage system NAND FLASH MEMORIES

NAND Flash memories are subjected to a progressive wear-out due to endurance (program and erase cycles i.e., P/E cycles) and data retention

Endurance in 1X MLCRetention in 1X MLC(@ 10 kP/E Cycles)

A direct indicator of such wear-out is the RBER

Solution: To improve RBER figures thus lowering the percentage of uncorrectable pages, NAND flash vendors have introduced the Read Retry (RR) algorithm

Endurance in 1X MLCRetention in 1X MLC(@ 10 kP/E Cycles)

Take away: when RR is used, memory read time increases with respect to normal read time (up to 256% in tested device)

Problem: Increasing RBERs result in higher probability of erroneously decoding the bits read in a page (percentage of uncorrectable pages).

Take away 1: It is tightly coupled with the ECC’s correction capabilitiesTake away 2: As soon as it becomes higher than zero the whole SSD is

considered no longer reliable

Endurance in 1X MLCRetention in 1X MLC(@ 10 kP/E Cycles)

Take away: To improve SSD’s reliability advanced ECCs must be used

When looking at SSDs where Quality of Service (QoS) standards must be met, exploiting RR techniques could introduce a performance/reliability trade-off for the whole system

SSD’s bandwidth and latencies have been collected by means of the SSDExplorer co-simulation framework

RBER and percentage of uncorrectable pages

characteristics

host system command submission/completion

timings

Queue depth(1 Thread) Real Device SSDExplorer Matching

Read BW Read BW BW Delta

32 642691 KB/s 626112 KB/s -2.58%64 813615 KB/s 822295 KB/s 1.07%128 861639 KB/s 870096 KB/s 0.98%

Hardware/Software Co-simulation

Endurance in 1X MLC

Simulated architecture: 8 Channels, 4 targets per channel, 512GBECC: multi-threaded BCH 100bit/4320Bytes

Retention in 1X MLC(@ 10 kP/E Cycles)

SSD’s read latency distributions: minimum, 25th percentile, median, 75th percentile and maximum were measured.

An enterprise class QoS of 5ms has been set as reference

Conclusions:1. RR is able to enhance both endurance and data retention. In measured device endurance and retention were extended up to 31% and 300% respectively.2. SSD’s read performance and latencies were heavily impacted by RR. If QoS has to be guaranteed actual endurance and data retention extensions

provided by RR settle around 10% and 7% respectively.

SSD’s read bandwidth.

Endurance in 1X MLCRetention in 1X MLC(@ 10 kP/E Cycles)

QoS limit marks a 10% performance degradation compared to the beginning of life (endurance) and beginning of retention time.

ECC/SSD fail point (ECCTH)ECC/SSD fail point (ECCTH)