A VtBlock represents a venti data block. Blocks stored on a venti server, called global blocks, are named by the SHA1 hash of their contents. This hash is recorded as the block's score. Such blocks are immutable. The cache also stores mutable blocks that have not yet been written to a venti server. These blocks are called local blocks, and have special scores that are 16 zero bytes followed by a 4-byte big-endian address. The address is an index into the internal set of cache blocks.

The user-visible contents of a VtBlock are data, a pointer to the data; type, the venti block type; score, the block's score; and addr, the block's cache address.

Vtcachealloc allocates a new cache using the client connection z (see venti-conn(2) and venti-client(2)), with room for nblocks of maximum block size blocksize .

Vtcachefree frees a cache and all the associated blocks.

Vtcacheblocksize returns the cache's maximum block size.

Vtglobaltolocal returns the local address corresponding to the given local score. If passed a global score, vtglobaltolocal returns the special constant NilBlock (~0). Vtlocaltoglobal is the opposite, setting score to the local score for the cache address local.

Vtcacheallocblock allocates a new local block with the given type.

Vtcachelocal retrieves the local block at address addr from the cache. The given type must match the type of the block found at addr.

Vtcacheglobal retrieves the block with the given score and dtype from the cache, consulting the Venti server if necessary. If passed a local score, vtcacheglobal invokes vtcachelocal appropriately.

The block references returned by vtcacheallocblock, vtcachelocal, and vtcacheglobal must be released when no longer needed. Vtblockput releases such a reference.

It is occasionally convenient to have multiple variables refer to the same block. Vtblockduplock increments the block's reference count so that an extra vtblockput will be required in order to release the block.

Vtblockwrite writes a local block to the Venti server, changing the block to a global block. It calls the cache's write function to write the block to the server. The default write function is vtwrite (see venti-client(2)); vtsetcachewrite sets it. Vtsetcachewrite is used by clients to install replacement functions that run writes in the background or perform other additional processing.

Vtblockcopy copies a block in preparation for modifying its contents. The old block may be a local or global block, but the new block will be a local block.

The cache only evicts global blocks. Local blocks can only leave the cache via vtblockwrite, which turns them into global blocks, making them candidates for eviction.

If a new cache block must be allocated (for vtcacheallocblock, vtcachelocal, vtcacheglobal, or vtblockcopy), but the cache is filled (with local blocks and blocks that have not yet been released with vtblockput), the library prints the score and reference count of every block in the cache and then aborts. A full cache indicates either that the cache is too small, or, more commonly, that cache blocks are being leaked.