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.SH
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The server processes
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.PP
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The main file system algorithm is a set
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of identical processes
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named
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.CW srv
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that honor the
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9P protocol.
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Each file system process waits on
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a message queue for an incoming request.
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The request contains a 9P message and
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the address of a reply queue.
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A
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.CW srv
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process parses the message,
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performs pseudo-disk I/O
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to the corresponding file system block device,
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formulates a response,
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and sends the
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response back to the reply queue.
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.PP
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The unit of storage is a
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logical block
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(not physical sector) of data on a device:
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.Ex
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.TA 0.5i 1i 1.5i 2i 2.5i 3i 3.5i 4i 4.5i 5i 5.5i
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	enum
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	{
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		RBUFSIZE = 8*1024
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	};
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	typedef vlong Off;
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	typedef
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	struct
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	{
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		short	pad;
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		short	tag;
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		Off	path;
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	} Tag;
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	enum
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	{
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		BUFSIZE = RBUFSIZE - sizeof(Tag)
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	};
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	typedef
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	struct
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	{
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		uchar	data[BUFSIZE];
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		Tag	tag;
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	} Block;
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.Ee
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All devices are idealized as a perfect disk
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of contiguously numbered blocks each of size
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.CW RBUFSIZE .
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Each block has a tag that identifies what type
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of block it is and a unique id of the file or directory
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where this block resides.
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The remaining data in the block depends on
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what type of block it is.
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.PP
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The
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.CW srv
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process's main data structure is the directory entry.
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This is the equivalent of a UNIX i-node and
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defines the set of block addresses that comprise a file or directory.
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Unlike the i-node,
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the directory entry also has the name of the
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file or directory in it:
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.Ex
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	enum
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	{
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		NAMELEN = 56,
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		NDBLOCK = 6,
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		NIBLOCK = 4,
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	};
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.Ee
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.Ex
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	typedef
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	struct
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	{
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		char	name[NAMELEN];
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		short	uid;
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		short	gid;
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		ushort	mode;
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		short	wuid;
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		Qid	qid;
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		Off	size;
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		Off	dblock[NDBLOCK];
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		Off	iblocks[NIBLOCK];
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		long	atime;
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		long	mtime;
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	} Dentry;
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.Ee
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Each directory entry holds the file or directory
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name, protection mode, access times, user-id, group-id, and addressing
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information.
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The entry
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.CW wuid
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is the user-id of the last writer of the file
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and
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.CW size
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is the size of the file in bytes.
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The addresses of the first 6
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blocks of the file are held in the
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.CW dblock
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array.
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If the file is larger than that,
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an indirect block is allocated that holds
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the next
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.CW BUFSIZE/sizeof(Off)
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block addresses of the file.
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The indirect block address is held in
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.CW iblocks[0] .
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If the file is larger yet,
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then there is a double indirect block that points
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at indirect blocks.
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The double indirect address is held in
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.CW iblocks[1]
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and can point at another
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.CW (BUFSIZE/sizeof(Off))\u\s-2\&2\s+2\d
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blocks of data.
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This is extended through a quadruple indirect block at
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.CW iblocks[3]
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but the code is now parameterised to permit easily changing the
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number of direct blocks and the depth of indirect blocks,
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and also the maximum size of a file name component.
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The maximum addressable size of a file is
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therefore 7.93 petabytes at a block size of 8k,
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but 7.98 exabytes (just under $2 sup 63$ bytes) at a block size of 32k.
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File size is restricted to $2 sup 63 - 1$ bytes in any case
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because the length of a file is maintained in a
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(signed)
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.I vlong .
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These numbers are based on
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.I fs64
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which has a block size of 8k and
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.CW sizeof(Off)
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is 8.
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.PP
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The declarations of the indirect and double indirect blocks
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are as follows.
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.Ex
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	enum
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	{
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		INDPERBUF = BUFSIZE/sizeof(Off),
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	};
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.Ee
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.Ex
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	typedef
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	{
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		Off	dblock[INDPERBUF];
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		Tag	ibtag;
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	} Iblock;
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.Ee
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.Ex
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	typedef
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	{
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		Off	iblock[INDPERBUF];
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		Tag	dibtag;
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	} Diblock;
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.Ee
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.PP
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The root of a file system is a single directory entry
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at a known block address.
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A directory is a file that consists of a list of
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directory entries.
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To make access easier,
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a directory entry cannot cross blocks.
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In
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.I fs64
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there are 47 directory entries per block.
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.PP
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The device on which the blocks reside is implicit
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and ultimately comes from the 9P
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.CW attach
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message that specifies the name of the
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device containing the root.