VtBlock, VtCache, vtblockcopy, vtblockduplock, vtblockput, vtblockwrite, vtcachealloc, vtcacheallocblock, vtcacheblocksize, vtcachefree, vtcacheglobal, vtcachelocal, vtcachesetwrite, vtglobaltolocal, vtlocaltoglobal - Venti block cache #include <u.h>

#include <libc.h>

#include <venti.h>

typedef struct VtBlock
{
 uchar *data;
 uchar type;
 uchar score[VtScoreSize];
 u32int addr;
 ...
} 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);
 DESCRIPTION
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 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.
 SOURCE
 /sys/src/libventi 
 SEE ALSO
 venti (2),  venti-client (2),  venti-conn (2),  venti-file (2),  venti (6)