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 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 Emelie there are 26 processes of 10 types:
number name function 15 srv Main file system server processes 1 rah Block read-ahead processes h'w'0'u'1 scp Sync process h'w'0'u'1 wcp WORM copy process h'w'0'u'1 con Console process h'w'0'u'1 ilo IL protocol process h'w'0'u'1 ilt IL timer process h'w'0'u'2 ethi Ethernet input process h'w'0'u'2 etho Ethernet output process h'w'0'u'1 flo Floppy disk process
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 block of data on a device:
enum
{
RBUFSIZE = 16*1024
};
typedef
struct
{
short pad;
short tag;
long path;
} Tag;
enum
{
BUFSIZE = RBUFSIZE - sizeof(Tag)
};
typedef
struct
{
uchar data[BUFSIZE];
Tag tag;
} Block;
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:
enum
{
NAMELEN = 28,
NDBLOCK = 6
};
typedef
struct
{
char name[NAMELEN];
short uid;
short gid;
ushort mode;
short wuid;
Qid qid;
long size;
long dblock[NDBLOCK];
long iblock;
long diblock;
long atime;
long mtime;
} Dentry;
The declarations of the indirect and double indirect blocks are as follows.
enum
{
INDPERBUF = BUFSIZE/sizeof(long),
};
typedef
{
long dblock[INDPERBUF];
Tag ibtag;
} Iblock;
typedef
{
long iblock[INDPERBUF];
Tag dibtag;
} Diblock;
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 Emelie there are 233 directory entries per block.
The device on which the blocks reside is implicit and ultimately comes from the 9P attach message that specifies the name of the device containing the root.
When the file server is booted, all of the unused memory is allocated to a block buffer pool. There are two major operations on the buffer pool. Getbuf will find the buffer associated with a particular block on a particular device. The returned buffer is locked so that the caller has exclusive use. If the requested buffer is not in the pool, some other buffer will be relabeled and the data will be read from the requested device. Putbuf will unlock a buffer and if the contents are marked as modified, the buffer will be written to the device before the buffer is relabeled. If there is some special mapping or CPU cache flushing that must occur in order for the physical I/O device to access the buffers, this is done between getbuf and putbuf. The contents of a buffer is never touched except while it is locked between getbuf and putbuf calls.
The file system server processes prevent deadlock in the buffers by always locking parent and child directory entries in that order. Since the entire directory structure is a hierarchy, this makes the locking well-ordered, preventing deadlock. The major problem in the locking strategy is that locks are at a block level and there are many directory entries in a single block. There are unnecessary lock conflicts in the directory blocks. When one of these directory blocks is tied up accessing the very slow WORM, then all I/O to dozens of unrelated directories is blocked.
The block device I/O system is like a protocol stack of filters. There are a set of pseudo-devices that call recursively to other pseudo-devices and real devices. The protocol stack is compiled from a configuration string that specifies the order of pseudo-devices and devices. Each pseudo-device and device has a set of entry points that corresponds to the operations that the file system requires of a device. The most notable operations are read, write, and size.
The device stack can best be described by describing the syntax of the configuration string that specifies the stack. Configuration strings are used during the setup of the file system. For a description see fsconfig(8). In the following recursive definition, D represents a string that specifies a block device.
For both w and r devices any of the configuration numbers can be replaced by an iterator of the form <N1-N2>. Thus
[w0.<2-6>]
c[w1.<0-5>.0]j(w6w5w4w3w2)l(<0-236>l<238-474>)
There are a set of file system processes, rah, that wait for messages consisting of a device and block address. When a message comes in, the process reads the specified block from the device. This is done by calling getbuf and putbuf. The purpose of this is the hope that these blocks will be used later and that they will reside in the buffer cache long enough not to be discarded before they are used.
The messages to the read-ahead processes are generated by the server processes. The server processes maintain a relative block mark in every open file. Whenever an open file reads that relative block, the next 110 block addresses of the file are sent to the read-ahead processes and the relative block mark is advanced by 100. The initial relative block is set to 1. If the file is opened and only a few bytes are read, then no anticipating reads are performed since the relative block mark is set to 1 and only block offset 0 is read. This is to prevent some fairly common action such as
file *
The cache/WORM (cw) driver is by far the largest and most complicated device driver in the file server. There are four devices involved in the cw driver. It implements a read/write pseudo-device (the cw-device) and a read-only pseudo-device (the dump device) by performing operations on its two constituent devices the read-write c-device and the write-once-read-many w-device. The block numbers on the four devices are distinct, although the cw addresses, dump addresses, and the w addresses are highly correlated.
The cw-driver uses the w-device as the stable storage of the file system at the time of the last dump. All newly written and a large number of recently used exact copies of blocks of the w-device are kept on the c-device. The c-device is much smaller than the w-device and so the subset of w-blocks that are kept on the c-device are mapped through a hash table kept on a partition of the c-device.
The map portion of the c-device consists of blocks of buckets of entries. The declarations follow.
enum
{
BKPERBLK = 10,
CEPERBK = (BUFSIZE - BKPERBLK*sizeof(long)) /
(sizeof(Centry)*BKPERBLK),
};
typedef
struct
{
ushort age;
short state;
long waddr;
} Centry;
typedef
struct
{
long agegen;
Centry entry[CEPERBK];
} Bucket;
Bucket bucket[BKPERBLK];
The following steps go into converting a w-address into a c-address. The bucket is found by
bucket_number = w-address % total_buckets
getbuf(c-device, bucket_offset + bucket_number/BKPERBLK);
The state variable in the entry is one of the following.
enum
{
Cnone = 0,
Cdirty,
Cdump,
Cread,
Cwrite,
Cdump1,
};
The Cwrite and Cdirty blocks are created and never removed. Unless something is done to convert these blocks, the c-device will gradually fill up and stop functioning. Once a day, or by command, a dump of the cw-device is taken. The purpose of a dump is to queue the writes that have been shunted to the c-device to be written to the w-device. Since the w-device is a WORM, blocks cannot be rewritten. Blocks that have already been written to the WORM must be relocated to the unused portion of the w-device. These are precisely the blocks with Cwrite state.
The dump algorithm is as follows: a) The tree on the cw-device is walked as long as the blocks visited have been modified since the last dump. These are the blocks with state Cwrite and Cdirty. It is possible to restrict the search to within these blocks since the directory containing a modified file must have been accessed to modify the file and accessing a directory will set its modified time thus causing the block containing it to be written. The directory containing that directory must be modified for the same reason. The tree walk is thus drastically restrained and the tree walk does not take much time. b) All Cwrite blocks found in the tree search are relocated to new blank blocks on the w-device and converted to Cdump state. All Cdirty blocks are converted to Cdump state without relocation. At this point, all modified blocks in the cw-device have w-addresses that point to unwritten WORM blocks. These blocks are marked for later writing to the w-device with the state Cdump. c) All open files that were pointing to modified blocks are reopened to point at the corresponding reallocated blocks. This causes the directories leading to the open files to be modified. Thus the invariant discussed in a) is maintained. d) The background dumping process will slowly go through the map of the c-device and write out all blocks with Cdump state.
The dump takes a few minutes to walk the tree and mark the blocks. It can take hours to write the marked blocks to the WORM. If a marked block is rewritten before the old copy has been written to the WORM, it must be forced to the WORM before it is rewritten. There is no problem if another dump is taken before the first one is finished. The newly marked blocks are just added to the marked blocks left from the first dump.
If there is an error writing a marked block to the WORM then the dump state is converted to Cdump1 and manual intervention is needed. (See the cwcmd mvstate command in fs(8)). These blocks can be disposed of by converting their state back to Cdump so that they will be written again. They can also be converted to Cwrite state so that they will be allocated new addresses at the next dump. In most other respects, a Cdump1 block behaves like a Cwrite block.
The scp process wakes up every ten seconds and issues writes to blocks in the buffer cache that have been modified. This is done automatically on important console commands such as halt and dump.
The wcp process also wakes up every ten seconds and tries to copy a dump block from the cache to the WORM. As long as there are dump blocks to copy and there is no competition for the WORM device, the copy will continue at full speed. Whenever there is competition for the WORM or there are no more blocks to copy, then the process will sleep ten seconds before looking again.
The HP WORM jukebox consists of 238 disks divided into 476 sides or platters. Platter 0 is the A side of disk 0. Platter 1 is the A side of the disk 1. Platter 238 is the B side of disk 0. On Emelie, the main file system is configured on both sides of the first 237 disks, platters 0-236 and 238-474.
The file server described so far waits for 9P protocol messages to appear in its input queue. It processes each message and sends the reply back to the originator. There are groups of processes that perform protocol I/O on some network or device and the resulting messages are sent to the file system queue.
There are two sets of processes ethi and etho that perform Ethernet input and output on two different networks. These processes send Ethernet messages to/from two more processes ilo and ilt that do the IL reliable datagram protocol on top of IP packets.
The last process in Emelie, con, reads the console and calls internal subroutines to executes commands typed. Since there is only one process, only one command can be executing at a time. See fs(8) for a description of the commands available at the console.