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ENERGY-EFFICIENCY AND STORAGE FLEXIBILITY IN THE BLUE FILE SYSTEM. E. B. Nightingale and J. Flinn University of Michigan. Key ideas. Reduce power consumption of portable devices Two big culprits Disk drive Wireless Integrate flash drives. Dynamic storage hierarchy.
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ENERGY-EFFICIENCY AND STORAGE FLEXIBILITYIN THE BLUE FILE SYSTEM E. B. Nightingale and J. Flinn University 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
Disk is powered up Disk is powered down Flash Flash Server Disk Disk Server Examples 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 Server Disk
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 Server Disk
Aggregating writes (II) • When it writes to a device, it flushes the whole queue Server Disk
Aggregating writes (III) • After the writes, each queue is empty Server Disk
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.
Updated for Fall 2012 Optimistic replication • 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
Updated for Fall 2012 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
If machines could talk (I) 99% of these requests are useless! I should contact the server each time I open any file Server Client
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
If machines could talk (III) What if I do not receive your callback? Though luck! Server Client
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 totau minutes before the time the client opened the file for the last time
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
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
Updated for Fall 2012 Applications Wolverine To BlueFS Server Up-call VFS Operations LinuxFile Cache BlueFSKernel Module USBStick CardFlash Micro-drive Local Disk LinuxKernel Blue FS architecture (II)
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
Applications Wolverine To BlueFS Server Up-call VFS Operations LinuxFile Cache BlueFSKernel Module USBStick CardFlash Micro-drive Local Disk LinuxKernel Blue FS architecture (II)
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 6000.0 Ten times faster than NFS, 16% faster than Coda WD
Adding a 16MB flash drive • Assumes that cache is fully loaded • 48% shorter interactive delay 48%, 25% less energy
Energy efficiency • Different benchmark run with half-full cache • 76% shorter interactive delay, 55% less energy than Coda
Adding a 16MB flash drive • Assumes that cache is fully loaded • 48% shorter interactive delay, 25% less energy
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 optimisticreplication, callbacks and disconnected mode operation