Keeping meta-data in files allows WAFL to write meta-data blocks anywhere on disk. This is the origin of the name WAFL, which stands for Write Anywhere File Layout. The write-anywhere design allows WAFL to operate efficiently with RAID by scheduling multiple writes to the same RAID stripe whenever possible to avoid the 4-to-1 write penalty that RAID incurs when it updates just one block in a stripe.

 

Keeping meta-data in files makes it easy to increase the size of the file system on the fly. When a new disk is added, the FAServer automatically increases the sizes of the meta-data files. The system administrator can increase the number of inodes in the file system manually if the default is too small.

 

Finally, the write-anywhere design enables the copy-on-write technique used by Snapshots. For Snapshots to work, WAFL must be able to write all new data, including meta-data, to new locations on disk, instead of overwriting the old data. If WAFL stored meta-data at fixed locations on disk, this would not be possible.

 

Most file systems keep track of free blocks using a bit map with one bit per disk block. If the bit is set, then the block is in use. This technique does not work for WAFL because many snapshots can reference a block at the same time. WAFL's block-map file contains a 32-bit entry for each 4 KB disk block. Bit 0 is set if the active file system references the block, bit 1 is set if the first Snapshot references the block, and so on. A block is in use if any of the bits in its block-map entry are set.

 

 

At time t1, the block-map entry is completely clear, indicating that the block is available. At time t2, WAFL allocates the block and stores file data in it. When Snapshots are created, at times t3 and t4, WAFL copies the active file system bit into the bit indicating membership in the Snapshot. The block is deleted from the active file system at time t5. This can occur either because the file containing the block is removed, or because the contents of the block are updated and the new contents are written to a new location on disk. The block can't be reused, however, until no Snapshot references it. In the Figure, this occurs at time t8 after both Snapshots that reference the block have been removed.