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 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 upDisk 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 for Fall 2012 Optimistic replication control puts data 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 for Fall 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 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

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 Kernel Module Wolverine Linux Kernel Linux File Cache USB Stick Card Flash Micro- drive Local Disk To BlueFS Server VFS Operatio ns Up- call Updated for Fall 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 Kernel Module Wolverine Linux Kernel Linux File Cache USB Stick Card Flash Micro- drive Local Disk To BlueFS Server VFS Operations Up-call

Performance Will use a standard benchmark –Andrews/AFS benchmark Will compare interactive delays and power 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

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 optimistic replication, callbacks and disconnected mode operation