The 64-bit Standalone Plan 9 File Server
Murray Hill, New Jersey 07974
This paper is a revision of Thompson’s The Plan 9 File Server, and describes the structure and the operation of the new 64-bit Plan 9 file servers. Some specifics apply to the 32-bit Plan 9 file server Emelie, which code is also the basis for the user-level file server kfs.
In 2004, Collyer created a 64-bit version of Thompson’s 32-bit file server, updating all file offsets, sizes and block numbers to 64 bits. In addition, triple- and quadruple-indirect blocks were implemented. File name components were extended from 27 to 55 bytes. This code is also the basis for the user-level file server cwfs(4).
The Plan 9 file server Emelie is the oldest piece of system software still in use on Plan 9. It evolved from a user-level program that served serial lines on a Sequent multi-processor. The current implementation is neither clean nor portable, but it has slowly come to terms with its particular set of cranky computers and devices.
The file server fs64 runs a revision of Emelie’s code with 64-bit file sizes, offsets and block numbers and indirect blocks from single to quadruple. Actually these are 63-bit values, since the type used is vlong (signed long long integer), but 63 bits should suffice for a little while.
The Plan 9 file system server is made from an ancient version of the Plan 9 kernel. The kernel contains process control, synchronization, locks, and some memory allocation. The kernel has no user processes or virtual memory.
The structure of the file system server is a set of kernel processes synchronizing mostly through message passing. In fs64 there are 27 processes of 11 types:
The server processes
The main file system algorithm is a set of identical processes named 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 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.
The unit of storage is a logical block (not physical sector) of data on a device:
RBUFSIZE = 8*1024
typedef vlong Off;
BUFSIZE = RBUFSIZE - sizeof(Tag)
All devices are idealized as a perfect disk of contiguously numbered blocks each of size 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.
The 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: