246 lines
4.9 KiB
Text
246 lines
4.9 KiB
Text
.TH VENTI-CACHE 2
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.SH NAME
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VtBlock, VtCache,
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vtblockcopy,
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vtblockdirty,
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vtblockduplock,
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vtblockput,
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vtblockwrite,
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vtcachealloc,
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vtcacheallocblock,
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vtcacheblocksize,
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vtcachefree,
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vtcacheglobal,
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vtcachelocal,
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vtcachesetwrite,
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vtglobaltolocal,
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vtlocaltoglobal \- Venti block cache
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.SH SYNOPSIS
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.ft L
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#include <u.h>
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.br
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#include <libc.h>
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.br
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#include <venti.h>
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.ta +\w'\fLxxxx 'u
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.PP
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.ft L
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.nf
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typedef struct VtBlock
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{
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uchar *data;
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uchar type;
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uchar score[VtScoreSize];
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u32int addr;
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...
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} VtBlock;
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.ta +\w'\fLVtBlock* 'u +\w'\fLxxxxxxxx'u
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.PP
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.B
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VtCache* vtcachealloc(VtConn *z, int blocksize, ulong nblocks);
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.PP
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.B
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void vtcachefree(VtCache *c);
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.PP
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.B
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u32int vtcacheblocksize(VtCache *c);
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.PP
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.B
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u32int vtglobaltolocal(uchar score[VtScoreSize])
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.br
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.B
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void vtlocaltoglobal(u32int local, uchar score[VtScoreSize])
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.PP
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.B
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VtBlock* vtcacheallocblock(VtCache *c, int type);
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.PP
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.B
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VtBlock* vtcachelocal(VtCache *c, u32int addr, int type);
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.PP
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.B
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VtBlock* vtcacheglobal(VtCache *c, uchar[VtScoreSize], int type);
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.PP
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.B
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void vtblockput(VtBlock *b);
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.PP
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.B
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void vtblockduplock(VtBlock *b);
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.PP
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.B
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int vtblockwrite(VtBlock *b);
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.PP
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.B
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void vtcachesetwrite(VtCache *c,
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.br
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.B
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int (*write)(VtConn*, uchar[VtScoreSize], uint, uchar*, int));
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.PP
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.B
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VtBlock* vtblockcopy(VtBlock *b);
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.PP
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.B
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int vtblockdirty(VtBlock *b);
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.SH DESCRIPTION
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These functions provide access to a simple in-memory
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cache of blocks already stored on a Venti server
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and blocks that will eventually be stored on a Venti server.
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.PP
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A
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.B VtBlock
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represents a venti data block.
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Blocks stored on a venti server,
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called
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.IR "global blocks" ,
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are named by the SHA1 hash of their contents.
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This hash is recorded as the block's
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.IR score .
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Such blocks are immutable.
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The cache also stores mutable blocks that have not yet been
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written to a venti server. These blocks are called
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.IR "local blocks" ,
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and have special scores that are 16 zero bytes
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followed by a 4-byte big-endian
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.IR address .
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The address is an index into the internal set of cache blocks.
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.PP
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The user-visible contents of a
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.B VtBlock
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are
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.BR data ,
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a pointer to the data;
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.BR type ,
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the venti block type;
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.BR score ,
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the block's score;
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and
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.BR addr ,
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the block's cache address.
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.PP
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.I Vtcachealloc
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allocates a new cache using the client connection
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.I z
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(see
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.IR venti-conn (2)
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and
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.IR venti-client (2)),
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with room for
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.I nblocks
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of maximum block size
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.IR blocksize .
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.PP
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.I Vtcachefree
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frees a cache and all the associated blocks.
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.PP
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.I Vtcacheblocksize
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returns the cache's maximum block size.
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.PP
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.I Vtglobaltolocal
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returns the local address corresponding to the given
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local
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.IR score .
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If passed a global score,
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.I vtglobaltolocal
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returns the special constant
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.B NilBlock
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.RB ( ~0 ).
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.I Vtlocaltoglobal
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is the opposite, setting
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.I score
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to the local score for the cache address
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.IR local .
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.PP
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.I Vtcacheallocblock
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allocates a new local block with the given
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.IR type .
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.PP
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.I Vtcachelocal
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retrieves the local block at address
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.I addr
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from the cache.
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The given
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.I type
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must match the type of the block found at
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.IR addr .
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.PP
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.I Vtcacheglobal
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retrieves the block with the given
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.I score
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and
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.I dtype
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from the cache, consulting the Venti server
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if necessary.
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If passed a local score,
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.I vtcacheglobal
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invokes
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.I vtcachelocal
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appropriately.
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.PP
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The block references returned by
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.IR vtcacheallocblock ,
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.IR vtcachelocal ,
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and
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.I vtcacheglobal
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must be released when no longer needed.
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.I Vtblockput
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releases such a reference.
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.PP
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It is occasionally convenient to have multiple variables
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refer to the same block.
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.I Vtblockduplock
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increments the block's reference count so that
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an extra
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.I vtblockput
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will be required in order to release the block.
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.PP
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.I Vtblockwrite
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writes a local block to the Venti server,
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changing the block to a global block.
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It calls the cache's
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.I write
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function
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to write the block to the server.
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The default
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.I write
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function is
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.I vtwrite
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(see
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.IR venti-client (2));
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.I vtsetcachewrite
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sets it.
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.I Vtsetcachewrite
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is used by clients to install replacement functions
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that run writes in the background or perform other
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additional processing.
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.PP
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.I Vtblockcopy
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copies a block in preparation for modifying its contents.
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The old block may be a local or global block,
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but the new block will be a local block.
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.PP
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The cache only evicts global blocks.
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Local blocks can only leave the cache via
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.IR vtblockwrite ,
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which turns them into global blocks, making them candidates for
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eviction.
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.PP
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If a new cache block must be allocated (for
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.IR vtcacheallocblock ,
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.IR vtcachelocal ,
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.IR vtcacheglobal ,
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or
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.IR vtblockcopy ),
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but the cache is filled (with local blocks and blocks that
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have not yet been released with
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.IR vtblockput ),
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the library prints the score and reference count of
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every block in the cache and then aborts.
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A full cache indicates either that the cache is too small,
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or, more commonly, that cache blocks are being leaked.
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.SH SOURCE
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.B /sys/src/libventi
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.SH SEE ALSO
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.IR venti (2),
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.IR venti-client (2),
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.IR venti-conn (2),
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.IR venti-file (2),
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.IR venti (6)
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