ref: 6b989beb2f870c46363c1feba1a25c59add41d5a
dir: /sys/doc/fs/p2/
.SH The server processes .PP The main file system algorithm is a set of identical processes named .CW srv that honor the 9P protocol. Each file system process waits on a message queue for an incoming request. The request contains a 9P message and the address of a reply queue. A .CW srv process parses the message, performs pseudo-disk I/O to the corresponding file system block device, formulates a response, and sends the response back to the reply queue. .PP The unit of storage is a logical block (not physical sector) of data on a device: .Ex .TA 0.5i 1i 1.5i 2i 2.5i 3i 3.5i 4i 4.5i 5i 5.5i enum { RBUFSIZE = 8*1024 }; typedef vlong Off; typedef struct { short pad; short tag; Off path; } Tag; enum { BUFSIZE = RBUFSIZE - sizeof(Tag) }; typedef struct { uchar data[BUFSIZE]; Tag tag; } Block; .Ee All devices are idealized as a perfect disk of contiguously numbered blocks each of size .CW RBUFSIZE . Each block has a tag that identifies what type of block it is and a unique id of the file or directory where this block resides. The remaining data in the block depends on what type of block it is. .PP The .CW srv process's main data structure is the directory entry. This is the equivalent of a UNIX i-node and defines the set of block addresses that comprise a file or directory. Unlike the i-node, the directory entry also has the name of the file or directory in it: .Ex enum { NAMELEN = 56, NDBLOCK = 6, NIBLOCK = 4, }; .Ee .Ex typedef struct { char name[NAMELEN]; short uid; short gid; ushort mode; short wuid; Qid qid; Off size; Off dblock[NDBLOCK]; Off iblocks[NIBLOCK]; long atime; long mtime; } Dentry; .Ee Each directory entry holds the file or directory name, protection mode, access times, user-id, group-id, and addressing information. The entry .CW wuid is the user-id of the last writer of the file and .CW size is the size of the file in bytes. The addresses of the first 6 blocks of the file are held in the .CW dblock array. If the file is larger than that, an indirect block is allocated that holds the next .CW BUFSIZE/sizeof(Off) block addresses of the file. The indirect block address is held in .CW iblocks[0] . If the file is larger yet, then there is a double indirect block that points at indirect blocks. The double indirect address is held in .CW iblocks[1] and can point at another .CW (BUFSIZE/sizeof(Off))\u\s-2\&2\s+2\d blocks of data. This is extended through a quadruple indirect block at .CW iblocks[3] but the code is now parameterised to permit easily changing the number of direct blocks and the depth of indirect blocks, and also the maximum size of a file name component. The maximum addressable size of a file is therefore 7.93 petabytes at a block size of 8k, but 7.98 exabytes (just under $2 sup 63$ bytes) at a block size of 32k. File size is restricted to $2 sup 63 - 1$ bytes in any case because the length of a file is maintained in a (signed) .I vlong . These numbers are based on .I fs64 which has a block size of 8k and .CW sizeof(Off) is 8. .PP The declarations of the indirect and double indirect blocks are as follows. .Ex enum { INDPERBUF = BUFSIZE/sizeof(Off), }; .Ee .Ex typedef { Off dblock[INDPERBUF]; Tag ibtag; } Iblock; .Ee .Ex typedef { Off iblock[INDPERBUF]; Tag dibtag; } Diblock; .Ee .PP The root of a file system is a single directory entry at a known block address. A directory is a file that consists of a list of directory entries. To make access easier, a directory entry cannot cross blocks. In .I fs64 there are 47 directory entries per block. .PP The device on which the blocks reside is implicit and ultimately comes from the 9P .CW attach message that specifies the name of the device containing the root.