VtBlock, VtCache, vtblockcopy, vtblockduplock, vtblockput, vtblock‐
write, vtcachealloc, vtcacheallocblock, vtcacheblocksize, vtcachefree,
vtcacheglobal, vtcachelocal, vtcachesetwrite, vtglobaltolocal, vtlocal‐
toglobal - Venti block cache
typedef struct VtBlock
VtCache* vtcachealloc(VtConn *z, int blocksize, ulong nblocks);
void vtcachefree(VtCache *c);
u32int vtcacheblocksize(VtCache *c);
u32int vtglobaltolocal(uchar score[VtScoreSize])
void vtlocaltoglobal(u32int local, uchar score[VtScoreSize])
VtBlock* vtcacheallocblock(VtCache *c, int type);
VtBlock* vtcachelocal(VtCache *c, u32int addr, int type);
VtBlock* vtcacheglobal(VtCache *c, uchar[VtScoreSize], int type);
void vtblockput(VtBlock *b);
void vtblockduplock(VtBlock *b);
int vtblockwrite(VtBlock *b);
void vtcachesetwrite(VtCache *c,
int (*write)(VtConn*, uchar[VtScoreSize], uint, uchar*, int));
VtBlock* vtblockcopy(VtBlock *b);
These functions provide access to a simple in-memory cache of blocks
already stored on a Venti server and blocks that will eventually be
stored on a Venti server.
A VtBlock represents a venti data block. Blocks stored on a venti
server, called global blocks, are named by the SHA1 hash of their con‐
tents. 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
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, set‐
ting 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
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 back‐
ground 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, vtcache‐
local, vtcacheglobal, or vtblockcopy), but the cache is filled (with
local blocks and blocks that have not yet been released with vtblock‐
put), 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
SEE ALSOventi(2), venti-client(2), venti-conn(2), venti-file(2), venti(6)VENTI-CACHE(2)