Information Technology Reference
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File system ngerprints. Some le systems, like Oracle's ZFS, include le
system fingerprints that provide a checksum across the entire file system in a
Denition: file system
fingerprint
way that allows ecient checks and updates when individual blocks are read
and written.
As illustrated in Figure 14.8 (a), all of ZFS's data structures are arranged
in a tree of blocks with a root node called the uberblock. At each internal node
of the tree, each reference to a child node includes both a pointer to and a
checksum of the child. Thus, the reference to any subtree includes a checksum
that covers all of that subtree's contents, and the uberblock holds a checksum
that covers the entire file system.
When ZFS reads data (i.e., leaves of the tree) or metadata (i.e., internal
nodes of the tree), it follows the pointers down the tree to find the right block
to read, computing a checksum of each internal or leaf block and comparing it
to the checksum stored with the block reference. Similarly, as Figure 14.8 (b)
illustrates, when ZFS writes a block, it updates the references from the updated
block to the uberblock so that each includes both the new checksum and (since
ZFS never updates data structures in place) new block pointer.
 
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