Editing Write Anywhere File Layout (section)

==Design==
[[File:Write Anywhere File Layout.png|thumb|WAFL inode structure, metadata stored alongside data]]
WAFL stores metadata, as well as data, in files; metadata, such as inodes and block maps indicating which blocks in the volume are allocated, are not stored in fixed locations in the file system. The top-level file in a volume is the inode file, which contains the [[inode]]s for all other files; the inode for the inode file itself, called the root inode, is stored in a block with a fixed location.  An inode for a sufficiently small file contains the file's contents; otherwise, it contains a list of pointers to file data blocks or a list of pointers to indirect blocks containing lists of pointers to file data blocks, and so forth, with as many layers of indirect blocks as are necessary, forming a tree of blocks.  All data and metadata blocks in the file system, other than the block containing the root inode, are stored in files in the file system.  The root inode can thus be used to locate all of the blocks of all files other than the inode file.<ref name="wafl-fs-design">{{cite conference|url=https://atg.netapp.com/wp-content/uploads/2000/01/file-system-design.pdf|title=File System Design for an NFS File Server Appliance|author1=Dave Hitz|author-link1=David Hitz|author2=James Lau|author3=Michael Malcolm|conference=USENIX Winter 1994|date=January 19, 1994}}</ref>

Main memory is used as a [[page cache]] for blocks from files.  When a change is made to a block of a file, the copy in the page cache is updated and marked dirty, and the difference is logged in non-volatile memory in a log called the ''NVLOG''.  If the dirty block in the page cache is to be written to permanent storage, it is not rewritten to the block from which it was read; instead, a new block is allocated on permanent storage, the contents of the block are written to the new location, and the inode or indirect block that pointed to the block in question is updated in main memory. If the block containing the inode, or the indirect block, is to be written to permanent storage, it is also written to a new location, rather than being overwritten at its previous position.  This is what the "Write Anywhere" in "Write Anywhere File Layout" refers to.<ref name="wafl-fs-design"/>

As all blocks, other than the block containing the root inode, are found via the root inode, none of the changes written to permanent storage are visible on permanent storage until the root inode is updated.  The root inode is updated by a process called a ''consistency point'', in which all dirty blocks not yet written to permanent storage are written to permanent storage, and a new root inode is written out, pointing to the blocks in the new version of the inode file.  At that point, all of the changes to the file system are visible on permanent storage, using the new root inode.  The NVLOG entries for changes that are now visible are discarded to make room for log entries for subsequent changes.  Consistency points are performed periodically or if the non-volatile memory is close to being full of log entries.<ref name="wafl-fs-design"/>

If the server crashes before all changes to a file system have been made visible in a consistency point, the changes that have not been made visible are still in the NVLOG; when the server reboots, it replays all entries in the NVLOG, again making the changes recorded in the NVLOG, so that they will not be lost.