Exploiting Flash for Energy Efficient Disk Arrays Shimin Chen (Intel Labs) Panos K. Chrysanthis (University of Pittsburgh) Alexandros Labrinidis (University of Pittsburgh)

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 2

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 3 [Li & Wang’04] [Pinheiro et al. ’06] [Yao & Wang’06]

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 4

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 5 Flash

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 6