VENTI-CACHE(2) VENTI-CACHE(2)
NAME
VtBlock, VtCache, vtblockcopy, vtblockdirty, vtblockduplock,
vtblockput, vtblockwrite, vtcachealloc, vtcacheallocblock,
vtcacheblocksize, vtcachefree, vtcacheglobal, vtcachelocal,
vtcachesetwrite, vtglobaltolocal, vtlocaltoglobal - Venti
block cache
SYNOPSIS
#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);
int vtblockdirty(VtBlock *b);
DESCRIPTION
Page 1 Plan 9 (printed 12/28/24)
VENTI-CACHE(2) VENTI-CACHE(2)
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 connec-
tion 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 neces-
sary. 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.
Page 2 Plan 9 (printed 12/28/24)
VENTI-CACHE(2) VENTI-CACHE(2)
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 glo-
bal 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)
Page 3 Plan 9 (printed 12/28/24)