Exploiting Flash for Energy Efficient Disk Arrays

1. Exploiting Flash for Energy Efficient Disk Arrays Shimin Chen (Intel Labs) PanosK. Chrysanthis(University of Pittsburgh) AlexandrosLabrinidis(University of Pittsburgh)

2. Motivation • Growing concern on data center energy consumption • Energy consumption of data storage: • Fastest annual growth among data center components • 20% between 2000 and 2006 [EPA report] • Goal: energy proportional data storage • i.e. Energy consumption system utilization • Challenging: HDD is dominant technology • HDD idle power is often 80% of active power • Transition to/from standby mode takes ~10 seconds • Could incur significant application slowdowns

3. Previous Approach:Exploit Redundancy and NVRAM • Most storage systems today employ redundancy • High reliability, availability, performance for applications • E.g. TPC-E requires redundancy in both data and logs • Idea: spin down disks containing redundant copies of data when system is under low load • Mirror-based (e.g., RAID 10): one disk active per mirror • Parity-based (e.g., RAID 5): use parity reconstruction • NVRAM (battery-backed RAM): • Maintain redundancy for writes • Spin up disks to apply buffered writes when NVRAM is full [Li & Wang’04] [Pinheiro et al. ’06] [Yao & Wang’06]

4. Limitations of Previous Approach • NVRAM size vs. HDD spin up/down wear cycles • Server-class HDDs: ~50,000 spin-up/down cycles • 5-year life time means ~1.1 spin-up/down per hour • NVRAM is expensive and thus small • Often hundreds of MB per disk array • Requires frequent disk spin up/down to flush NVRAM-buffered writes, reducing disk life time • Exploiting redundancy alone cannot achieve energy proportionality goal • E.g., for mirrored-disks, 50% disks are active when system is 1% utilized

5. Proposal 1: Exploit Flash as Write Buffer • Desirable properties of flash: • Nonvolatile: • Maintain redundancy • Much cheaper and much larger capacity: • Reduce spin-up/down cycles • Good performance for sequential writes and random reads • Can be efficiently used as write buffer under low load • Flash-based cache products with hundreds of GB capacity are already available for storage systems • Our proposal shares the flash resource: • Existing use: improve performance under high load • New use: reduce energy consumption under low load Flash

6. Proposal 2: Applications and Storage Collaborate to Further Save Energy • Energy proportionality goal implies spinning down more disks when system is under very low load • Not all data are immediately available • Potentially incurs large application slow down! • Applications (e.g., DBMS) and storage collaborate: • DBMS specifies hot and cold address ranges on RAID volume • DBMS chooses object temperature based on user requests and usage patterns in observed workloads • Storage guarantees data in hot address range are always available • Opportunities for data movement and replication in storage • DBMS can query storage to see if there will be a spin-up delay to access data in cold address ranges • DBMS may schedule work differently for tolerating such delays