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Write Anywhere File Layout
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Write Anywhere File Layout
The Write Anywhere File Layout (WAFL) is a proprietary file system that supports large, high-performance RAID arrays, quick restarts without lengthy consistency checks in the event of a crash or power failure, and growing the filesystems size quickly. It was designed by NetApp for use in its storage appliances like NetApp FAS, AFF, Cloud Volumes ONTAP and ONTAP Select.
Its author claims that WAFL is not a file system, although it includes one. It tracks changes similarly to journaling file systems as logs (known as NVLOGs) in dedicated memory storage device non-volatile random access memory, referred to as NVRAM or NVMEM. WAFL provides mechanisms that enable a variety of file systems and technologies that want to access disk blocks.
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 inodes 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.
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.
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.
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.
As discussed above, WAFL does not store data or metadata in pre-determined locations on disk. Instead it automatically places data using temporal locality to write metadata alongside user data in a way designed to minimize the number of disk operations required to commit data to stable disk storage using single and dual parity based RAID.
Using a data placement based on temporal locality of reference can improve the performance of reading datasets which are read in a similar way to the way they were written (e.g. a database record and its associated index entry), however it can also cause fragmentation from the perspective of spatial locality of reference. On spinning HDDs this does not adversely affect files that are sequentially written, randomly read, or are subsequently read using the same temporal pattern, but does affect sequential read after random write spatial data access patterns because of magnetic head could be only in one position at a time to read data from platter while fragmentation does no effect on SSD drives.
Hub AI
Write Anywhere File Layout AI simulator
(@Write Anywhere File Layout_simulator)
Write Anywhere File Layout
The Write Anywhere File Layout (WAFL) is a proprietary file system that supports large, high-performance RAID arrays, quick restarts without lengthy consistency checks in the event of a crash or power failure, and growing the filesystems size quickly. It was designed by NetApp for use in its storage appliances like NetApp FAS, AFF, Cloud Volumes ONTAP and ONTAP Select.
Its author claims that WAFL is not a file system, although it includes one. It tracks changes similarly to journaling file systems as logs (known as NVLOGs) in dedicated memory storage device non-volatile random access memory, referred to as NVRAM or NVMEM. WAFL provides mechanisms that enable a variety of file systems and technologies that want to access disk blocks.
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 inodes 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.
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.
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.
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.
As discussed above, WAFL does not store data or metadata in pre-determined locations on disk. Instead it automatically places data using temporal locality to write metadata alongside user data in a way designed to minimize the number of disk operations required to commit data to stable disk storage using single and dual parity based RAID.
Using a data placement based on temporal locality of reference can improve the performance of reading datasets which are read in a similar way to the way they were written (e.g. a database record and its associated index entry), however it can also cause fragmentation from the perspective of spatial locality of reference. On spinning HDDs this does not adversely affect files that are sequentially written, randomly read, or are subsequently read using the same temporal pattern, but does affect sequential read after random write spatial data access patterns because of magnetic head could be only in one position at a time to read data from platter while fragmentation does no effect on SSD drives.