ENERGY-EFFICIENCY%20AND%20STORAGE%20FLEXIBILITY%20IN%20THE%20BLUE%20FILE%20SYSTEM

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ENERGY-EFFICIENCY AND STORAGE FLEXIBILITYIN THE
BLUE FILE SYSTEM

• E. B. Nightingale and J. Flinn
• University of Michigan

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Key ideas

• Reduce power consumption of portable devices
• Two big culprits
• Disk drive
• Wireless
• Integrate flash drives

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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

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Examples

• Disk is powered up

• Disk is powered down

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

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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

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Write to all/read any

• All writes are propagated to all devices

Server
Disk
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Advantages

• Write all allows read any
• Future power savings
• Requires efficient writes
• Propagates all updates to the server
• Unless in disconnected mode

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Aggregating writes (I)

• Blue FS maintains one write queue per device

Server
Disk
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Aggregating writes (II)

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

Server
Disk
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Aggregating writes (III)

• After the writes, each queue is empty

Server
Disk
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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

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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

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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

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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

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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.

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Optimistic replication
Updated for Fall 2012

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

Optimistic replication control putsdata
availability above data consistency
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Callbacks (I)
Updated for Fall 2012

• 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

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If machines could talk (I)
99 of these requests are useless!
I should contact the server each time I open any
file
Server
Client
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If machines could talk (II)
You do not need to call me I promise to call you
back if anyone has modified the file
??
Server
Client
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If machines could talk (III)
What if I do not receive your callback?
Though luck!
Server
Client
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Callbacks (II)

• Callbacks can be lost!
• Client will call the server every tau minutes 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

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If machines could talk (IV)
Sure as long as you do not do it too frequently
I will contact you from time to time to check for
lost callbacks
Server
Client
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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

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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

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Accessing large files

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

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Deciding which device to use

• If decisions are made for each individual access
• Will never activate a disk thats 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

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Blue FS architecture (II)
Updated for Fall 2012

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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

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Blue FS architecture (II)
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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)

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Interactive delay
6000.0

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

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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!

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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