Write Anywhere File Layout
Encyclopedia
The Write Anywhere File Layout (WAFL) is a file layout that supports large, high-performance RAID arrays, quick restarts without lengthy consistency checks in the event of a crash
or power failure (though sometimes a WAFL check may be required which can take days), and growing the filesystems size quickly. It was designed by NetApp for use in its storage appliances
. WAFL is not a file system. WAFL provides mechanisms that enable a variety of file systems and technologies that want to access disk blocks
.
snapshots
allow users to recover files that have been accidentally deleted; they provide an online backup that can be accessed quickly. It is implemented similarly to that of a log-structured file system
. A special kind of snapshot that the filer uses internally called a consistency point allows WAFL to restart quickly in the event of an improper shutdown
. NetApp's Data ONTAP Release 7G operating system supports a read-write snapshot called FlexClone.
An important feature of WAFL is its support for both a Unix
-style file and directory model for NFS clients and a Microsoft Windows
-style file and directory model for CIFS
clients. WAFL also supports both security models, including a mode where different files on the same volume can have different security attributes attached to them. Unix can use either access control list
s (ACL) or a simple bitmask, whereas the more recent Windows model is based on access control lists. These two features make it possible to write a file to a CIFS type of networked filesystem and access it later via NFS from a Unix workstation.
As the name suggests Write Anywhere File Layout automatically fragments data using temporal locality to write metadata
alongside user data. This fragmentation
does not adversely affect files that are sequentially written to or randomly read from, but does affect sequential read after random write. Data ONTAP has the reallocate command as of 7G to perform scheduled and manual defragmentation
. Prior to 7G, the wafl scan reallocate command would need to be invoked from an advanced privilege level and could not be scheduled.
Crash (computing)
A crash in computing is a condition where a computer or a program, either an application or part of the operating system, ceases to function properly, often exiting after encountering errors. Often the offending program may appear to freeze or hang until a crash reporting service documents...
or power failure (though sometimes a WAFL check may be required which can take days), and growing the filesystems size quickly. It was designed by NetApp for use in its storage appliances
NetApp filer
In computer storage, NetApp filer, known also as NetApp Fabric-Attached Storage , or NetApp's network attached storage device are NetApp's offering in the area of Storage Systems. A FAS functions in an enterprise-class Storage area network as well as a networked storage appliance...
. WAFL is not a file system. WAFL provides mechanisms that enable a variety of file systems and technologies that want to access disk blocks
Block (data storage)
In computing , a block is a sequence of bytes or bits, having a nominal length . Data thus structured are said to be blocked. The process of putting data into blocks is called blocking. Blocking is used to facilitate the handling of the data-stream by the computer program receiving the data...
.
Features
One of WAFL's most salient features is the snapshot, or read-only copy of the file system. Zero-copyZero-copy
"Zero-copy" describes computer operations in which the CPU does not perform the task of copying data from one memory area to another. This is most often used to save on processing power and memory use when sending files over a network.- Principle :...
snapshots
Snapshot (computer storage)
In computer systems, a snapshot is the state of a system at a particular point in time. The term was coined as an analogy to that in photography. It can refer to an actual copy of the state of a system or to a capability provided by certain systems....
allow users to recover files that have been accidentally deleted; they provide an online backup that can be accessed quickly. It is implemented similarly to that of a log-structured file system
Log-structured file system
A log-structured filesystem is a file system design first proposed in 1988 by John K. Ousterhout and Fred Douglis. Designed for high write throughput, all updates to data and metadata are written sequentially to a continuous stream, called a log...
. A special kind of snapshot that the filer uses internally called a consistency point allows WAFL to restart quickly in the event of an improper shutdown
Shutdown (computing)
To shut down or power off a computer is to remove power from a computer's main components in a controlled way. After a computer is shut down, main components such as CPUs, RAM modules and hard disk drives are powered down, although some internal components, such as an internal clock, may retain...
. NetApp's Data ONTAP Release 7G operating system supports a read-write snapshot called FlexClone.
An important feature of WAFL is its support for both a Unix
Unix
Unix is a multitasking, multi-user computer operating system originally developed in 1969 by a group of AT&T employees at Bell Labs, including Ken Thompson, Dennis Ritchie, Brian Kernighan, Douglas McIlroy, and Joe Ossanna...
-style file and directory model for NFS clients and a Microsoft Windows
Microsoft Windows
Microsoft Windows is a series of operating systems produced by Microsoft.Microsoft introduced an operating environment named Windows on November 20, 1985 as an add-on to MS-DOS in response to the growing interest in graphical user interfaces . Microsoft Windows came to dominate the world's personal...
-style file and directory model for CIFS
Server Message Block
In computer networking, Server Message Block , also known as Common Internet File System operates as an application-layer network protocol mainly used to provide shared access to files, printers, serial ports, and miscellaneous communications between nodes on a network. It also provides an...
clients. WAFL also supports both security models, including a mode where different files on the same volume can have different security attributes attached to them. Unix can use either access control list
Access control list
An access control list , with respect to a computer file system, is a list of permissions attached to an object. An ACL specifies which users or system processes are granted access to objects, as well as what operations are allowed on given objects. Each entry in a typical ACL specifies a subject...
s (ACL) or a simple bitmask, whereas the more recent Windows model is based on access control lists. These two features make it possible to write a file to a CIFS type of networked filesystem and access it later via NFS from a Unix workstation.
As the name suggests Write Anywhere File Layout automatically fragments data using temporal locality to write metadata
Metadata
The term metadata is an ambiguous term which is used for two fundamentally different concepts . Although the expression "data about data" is often used, it does not apply to both in the same way. Structural metadata, the design and specification of data structures, cannot be about data, because at...
alongside user data. This fragmentation
Fragmentation (computer)
In computer storage, fragmentation is a phenomenon in which storage space is used inefficiently, reducing storage capacity and in most cases reducing the performance. The term is also used to denote the wasted space itself....
does not adversely affect files that are sequentially written to or randomly read from, but does affect sequential read after random write. Data ONTAP has the reallocate command as of 7G to perform scheduled and manual defragmentation
Defragmentation
In the maintenance of file systems, defragmentation is a process that reduces the amount of fragmentation. It does this by physically organizing the contents of the mass storage device used to store files into the smallest number of contiguous regions . It also attempts to create larger regions of...
. Prior to 7G, the wafl scan reallocate command would need to be invoked from an advanced privilege level and could not be scheduled.
External links
- Network Appliance: File System Design for an NFS File Server Appliance - Method for maintaining consistent states of a file system and for creating user-accessible read-only copies of a file system - October 6, 1998