Managing Storage Space in a Flash and Disk Hybrid Storage System

Managing Storage Space

in a Flash and Disk

Hybrid Storage System

Xiaojian Wu, and A. L. Narasimha Reddy

Dept. of Electrical and Computer Engineering

Texas A&M University

IEEE International Symposium on

Modeling, Analysis & Simulation of Computer and Telecommunication Systems , 2009

Outline

• Introduction

• Related Work

• Proposed Scheme

• Evaluation

• Conclusion

Introduction (1/4)

• To build large flash-only storage system is too expensive

– employ both flash and magnetic disk as a hybrid storage system

• Different characteristics

– write to flash can take longer than magnetic disk drives while

read can finish faster

– flash have a limit on the number of times a block can be written

– magnetic disks typically perform better with larger file sizes

• Data placement, retrieval, scheduling and buffer

management algorithms

needs to be revisited

for the

hybrid storage system

The disk drive is more efficient for larger reads and writes!

Introduction (3/4)

• Requests experience different performance at different

devices based on the

request type

(read or write)

and

Introduction (4/4)

• Managing the space across the devices in a hybrid

system should be adaptable to changing device

characteristics

• issues

– allocation

– data redistribution or migration

• Proposing a

measurement-driven approach

to

migration to address these issues

– observe the access characteristics of individual blocks and consider migrating individual blocks

Related Work

• HP’s AutoRAID system considered data migration

between a mirrored device and a RAID device

– migrate hot data to faster devices and cold data to

slower devices

– improve the access times of hot data by keeping it

local to faster devices

– when data sets are larger than the capacity of faster

devices in such systems,

thrashing

may occur

Proposed Scheme (1/5)

• pool the storage space across flash and disk drives and make it

appear like a single larger device to the file system

• maintain an

indirection map

, containing mappings of logical to

physical addresses, to allow blocks to be flexibly assigned to

different devices

– when data is migrated, indirection map needs to be updated – to reduce the cost, consider migration at a unit larger than a

Proposed Scheme (2/5)

• keep track of access behavior of a block by maintaining two counters, for Read and Write accesses

– use 2 bytes for keeping track of read/write frequency separately per chunk (64KB or larger)

• about 32KB per 1GB of storage

– a block can be considered for migration or relocation only after receiving a minimum number of accesses

• for observing sufficient access history

– block access counters are initialized to zero on boot-up and after migration

• Every time a request is served by the device, keep track of the request response time at that device

– maintain both read and write performance separately – exponential average of the device performance:

average response time = 0.99 * previous average + 0.01 * current sample

Proposed Scheme (4/5)

• For each device i, keep track of the read

r

• Determine whether to migration

– Given a block j’s read/write access history through its access counters R_j and

W_j and the device response times

– current cost of accessing block j in its current device i:

C

= (

R

* r

+

W

* w

) / (

R

+

W

)

– compare with a block with similar access patterns at another

device k, C_jk

– if

C

> (1+

δ

)*C

Proposed Scheme (5/5)

• Employ a token scheme to control the rate of migration • potential cost of a block migrated form device i to device k:

r

+

w

– only consider blocks are currently being read or written to the device, as part of normal I/O activity, to reduce the cost

• Strategy in choosing which block to migrate

– maintain a cache of recently accessed blocks

– whenever a migration token is generated, migrate a block from this cached list to benefit the most active blocks

• Migration is carried out in blocks or chunks of 64KB or larger

– larger block size increases migration costs, reduces the size of the indirection map,

Evaluation (1/7)

• NFS server

– Intel Pentium Dual Core 3.2 GHz processor – 1GB main memory

– magnetic disk:

one 7200RPM, 250G SAMSUNG SATA disk (SP2504C) – flash disk drives:

• a 16GB Transcend SSD (TS16GSSD25S-S) • a 32GB MemoRight GT drive

– Fedora 9 with a 2.6.21 kernel – Ext2 file system

• 3 Workloads

– SPECsfs 3.0

• file system workloads, read/write ratio about 1:4

– Postmark

• typical access patterns in an email server

– IOzone

• create controlled workloads at the storage system, control the read/write ratio from 100%, 75%, 50%, 25%, and 0%

• 4 policies

– FLASH-ONLY – MAGNETIC-ONLY – STRIPING

• data is striped on both flash and magnetic disk

– STRIPING-MIGRATION

• data is striped on and migrated across both disks

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(a) benefit from data redistribution matches the read/write characteristics of block to the device performance

(b) succeed in redistributing write-intensive blocks to the magnetic disk

Evaluation (2/7)

Evaluation (3/7)

Using δ = 1 and chunk size of 64KB in all the following experiments

if C_ji > (1+δ)*C_jk,

Evaluation (4/7)

2-HARDDISK STRIPING:

data is striped on two HDD and no migration is employed

Transcend 16G (slower) MemoRight 32G (faster)

(a)2-harddisk striping outperforms hybrid drive on both saturation point and response time

(b) hybrid drive achieves nearly 50% higher throughput saturation point

Using IOzone to create 100% writes to 75%, 50%, 25%, 0% write workloads

2-HARDDISK STRIPING: data is striped on two HDD and no migration is employed

STRIPING: data is striped on both flash and magnetic disk (Transcend-base hybrid drive)

STRIPING-MIGRATION: data is striped on and migrated across both disks

the read/write characteristics of the workload have a critical impact on the hybrid system

Evaluation (6/7)

• migration improves the transaction rate, read/write throughputs in both the hybrid systems by about 10%

• Transcend-based hybrid system can not compete with 2-HDD system • MemoRight-based hybrid system outperforms the 2-HDD system by roughly about 10-17%

Evaluation (7/7)

 Migration-1: consider only read/write characteristics

 Migration-2: request size is also considered

• if < 64KB, based on the read/write request pattern

• if > 64KB, allow to exploit the gain from striping data across both the devices

 For MemoRight-Hybrid

•Migration-1 improves performance over striping by about 7% • Migration-2 improves about 20% on average

 For Transcend-Hybrid

• the improvement of both migration policies is not as much • can not match the performance of the 2-HDD system

it shows that both read/write and request size patterns can be exploited to improve performance

Conclusion

• proposed a measurement-driven migration

strategy for managing storage space in a hybrid

system to exploit the performance asymmetry

• extract the read/write access patterns and

request size patterns of different blocks and

matches them with the read/write advantages of

different devices

• The results indicate that the proposed approach

can improve the performance of the system