Hitz et al.

“ Creating a clone in the prior art can use up as much as 32 MB on a 1 GB disk. The prior art requires 256 MB of disk space on a 1 GB disk (for 4 KB blocks) to keep eight clones of the file system. Thus, the prior art cannot use large numbers of clones to prevent loss of data. Instead it is used to facilitate backup of the file system onto an auxiliary storage means other than the disk drive, such as a tape backup device. Clones are used to backup a file system in a consistent state at the instant the clone is made. ”

“ In general, data on disk is not overwritten in the WAFL file system so as to protect data stored on disk. The only exception to this rule is atime overwrites for an inode as illustrated in FIGS. 23A-23B. When an "atime overwrites" occurs, the only data that is modified in a block of the inode file is the atime of one or more of the inodes it contains and the block is rewritten in the same location. This is the only exception in the WAFL system; otherwise new data is always written to new disk locations. ”

“ A block number is 4-bytes long. Use of block numbers having the same level of indirection in an inode better facilitates recursive processing of a file. FIG. 3 is a block diagram illustrating an on-disk inode 310. The on-disk inode 310 is comprised of standard inode information 310A and 16 block number entries 310B having the same level of indirection. The inode information 310A comprises information about the owner of a file, permissions, file size, access time, etc. that are well-known to a person skilled in the art. ”

“ When a system crash occurs between fsinfo block updates, each copy of the fsinfo block 1810 and 1870 has a self consistent checksum (not shown in the diagram) , but one of the generation numbers 1810D or 1870D has a higher value. ”

“ WAFL does this by keeping two identical copies of the fsinfo structure 1610 and 1612 containing the root inode 1610B and 1612B. During updating of the root inode 1610B and 1612B, a first copy of the fsinfo structure 1610 is written to disk, and then the second copy of the fsinfo structure 1612 is written. A checksum 1610C and 1612C in the fsinfo structure 1610 and 1612, respectively, is used to detect the occurrence of a system crash that corrupts one of the copies of the fsinfo structure 1610 or 1612, each containing a copy of the root mode, as it is being written to disk. ”

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“ Schwartz, Allan M., et al., LFS--A Local File System for Multiprocessor NFS Network Servers, Auspex Technical Report 4, .COPYRGT. 1989 Auspex Systems, Inc. Silberschatz, Abraham, et al., Operating Concepts, Ch 3.7: Interprocess Communication, pp. 127-147, .COPYRGT. 1988 Addison-Wesley Publishing Company, Inc. Stern, Hal, Managing NFS and NIS, NFS Daemons, .COPYRGT. 1991 O'Reilly & Associates, Inc. ”