ENERGY-EFFICIENCY AND STORAGE FLEXIBILITY

IN THE BLUE FILE SYSTEM

E. B. Nightingale and J. FlinnUniversity of Michigan

Key ideas

• Reduce power consumption of portable devices– Two big culprits

• Disk drive• Wireless

• Integrate flash drives

Dynamic storage hierarchy

• We should power down drives when they are idle– But this causes powering up delays

• To mask these delays introduce a dynamic storage hierarchy – Includes flash drives and remote server– Takes into consideration state of local disk drive

Examples

• Disk is powered up • Disk is powered down

Flash

Disk

Server

Flash

Server

Disk

The storage hierarchy depends on the status of the hard disk.

Incorporating flash drives

• To satisfy read requests, Blue FS looks first in the flash drive

• Implies– A write all/read any policy– Treating flash drives as caches

• Need a quick way to query flash drive contents

More on caches

• Unit of caching is block

• Can cache some blocks of a file but not all– Must maintain information on validity and

currency of each cached block• See details in paper

Write to all/read any

• All writes are propagated to all devices

Disk

Server

Advantages

• Write all allows read any– Future power savings

• Requires efficient writes– Propagates all updates to the server

• Unless in disconnected mode

Aggregating writes (I)

• Blue FS maintains one write queue per device

Disk

Server

Aggregating writes (II)

• When it writes to a device, it flushes the whole queue

Disk

Server

Aggregating writes (III)

• After the writes, each queue is empty

Disk

Server

More on device queues

• Device queues share among themselves all common blocks– Save space

• Blocks in device queues can be accessed through a hashing scheme– Always access most recent version of a block

even when it is not yet saved on any device

Why?

• Makes a lot of sense for disk drive and remote server– Power up disk, do a few updates, then power

down– Power up wireless connection, send a few

updates then power down• Delaying updates allows BlueFS to coalesce

multiple updates to the same block

Using flash drives as caches

• Small flash drives cannot contain whole contents of file system– Especially true in 2004!

• Must have a fast way to find whether a file is cached or not– Enode

E-node

• Captures all the information Blue FS has about the validity of an object

• E-nodes are– Hashed by file id– Stored in an e-node cache managed by LRU

replacement policy• Default size is one MB

Cache consistency

• Uses– Optimistic replica control (Coda)– Callbacks (AFS)

• Changes– Blue FS maintains callbacks on a per-device

basis, instead of on a per-client basis– Server queues invalidation messages when

a device is disconnected.

Optimistic replication

• Optimistic replication control protocols allow access in disconnected mode– Tolerate temporary inconsistencies– Promise to detect them later– Provide much higher data availability

Updated forFall 2012

Optimistic replication control putsdata availability above data consistency

Callbacks (I)

• When a client opens a file for the first time, server promises to notify it whenever it receives a new version of the file from any other client– Promise is called a callback

• Relieves the server from having to answer a call from the client every time the file is opened– Significant reduction of server workload

Updated forFall 2012

If machines could talk (I)

Client

I should contact the server each time I open

any file

Server

99% of these requests are useless!

If machines could talk (II)

Client

??

Server

You do not need to call me:I promise to call you back if anyone has modified the file

If machines could talk (III)

Client

What if I do not receive your

callback?

Server

Though luck!

Callbacks (II)

• Callbacks can be lost!

– Client will call the server every tauminutes to check whether it received all callbacks it should have received

– Cached copy is only guaranteed to reflect the state of the server copy up to tau minutes before the time the client opened the file for the last time

If machines could talk (IV)

Client

I will contact you from time to time to check

for lost callbacks

Server

Sure as long as you do not do it too frequently

Disconnected mode

• Like Coda, Blue FS works in disconnected mode

• User has access to all files cached– On local disk drive– On flash drive if any

• Can even specify that some files must always be cached– Affinity

Disconnected mode (II)

• When a device is disconnected, server queues all callbacks that could not be delivered to the device– Speeds up reintegration

• If inconsistencies are discovered at reconnection time, will run a resynchronization process

Accessing large files

• When disk is powered down, Blue FS get first file blocks from remote server

Deciding which device to use

• If decisions are made for each individual access– Will never activate a disk that’s powered down

• Blue FS uses ghost hints– Measure penalty for not using a disk that is

powered down for a given access– When sum of ghosts hints exceeds the cost of

powering up the disk, disk is powered up

Blue FS architecture (II)

Applications

BlueFS KernelModule

Wolverine

Linux Kernel

Linux File Cache

USBStick

CardFlash

Micro-drive

Local Disk

To BlueFS Server

VFS Operatio

ns

Up-call

Updated forFall 2012

Blue FS architecture (I)

• Most of Blue FS functionality is implemented in a user-level process – Wolverine

• Kernel-resident part of Blue FS intercepts I/O calls that have to be forwarded to Wolverine

Blue FS architecture (II)

Applications

BlueFS KernelModule

Wolverine

Linux Kernel

Linux File Cache

USBStick

CardFlash

Micro-drive

Local Disk

To BlueFS Server

VFS Operations

Up-call

Performance

• Will use a standard benchmark– Andrews/AFS benchmark

• Will compare interactive delays andpower consumption against– NFS – Coda with synchronous server updates (Coda)– Coda with asynchronous server updates

(Coda WD)

Interactive delay

Ten times faster than NFS, 16% faster than Coda WD

6000.0

Adding a 16MB flash drive

• Assumes that cache is fully loaded• 48% shorter interactive delay 48%, 25% less energy

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Energy efficiency

• Different benchmark run with half-full cache• 76% shorter interactive delay, 55% less energy than Coda

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Adding a 16MB flash drive

• Assumes that cache is fully loaded• 48% shorter interactive delay, 25% less energy

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Discussion

• Comparison with NFS is unfair– NFS was designed for LANs, not WANs!

• Comparison with Coda is fair but– Coda did not incorporate flash drives

• Did not exist then!– Coda did not try to minimize power

consumption• Portable devices did not exist then!

Conclusion

• Blue FS updates Coda by taking into consideration the arrival of– Wireless networks (near ubiquitous access)– Portable devices (limited autonomy)– Flash drives

• Retains Coda features such as optimistic replication, callbacks and disconnected mode operation