20 File Systems

In most computer systems more than one peripheral storage device is used for the storage of files. It is now necessary to discuss a number of matters pertaining to the management by UNIX of the whole set of files and file storage devices. First, some definitions:

file system:

an integrated collection of files with a hierarchical system of directories recorded on a single block oriented storage device;

storage device:

a device which can store information (especially disk pack or DECtape, etc.);

access device:

a mechanism for transferring information to or from a storage device;

a storage device

is only accessible if it is inserted in an access device. In this situation reference to the storage device is made via a reference to the access devce;

a storage device

is acceptable as a file system volume if:

(a)

information is recorded as addressable blocks of 512 characters each, which can be independently read or written.

(Note IBM compatible magnetic tape does not satisfy this condition.);

(b)

the information recorded on the device satisfies certain consistency criteria:

block #1 is formatted as a “super block” (see below);

blocks #2 to #(n+1) (where n is recorded in the “super block”) contain an “inode” table which references all files recorded on the storage device, and does not reference any other files;

directory files recorded on the storage device reference all, and only, files on the same storage device, i.e. a file system volume constitutes a self-contained set of files, directories and “inode” table;

a file system volume

is mounted if the presence of the storage device in an access device has been formally recognised by the operating system.

20.1 The ‘Super Block’ (5561)

The “super block” is always recorded as block #l on the storage device. (Block #0 is always ignored and is available for miscellaneous uses not necessarily concerned with UNIX.)

The “super block” contains information used in allocating resources, viz. the storage blocks and the entries in the “inode” table recorded on the file system. While the file system volume is mounted a copy of the “super block” is maintained in core and updated there. To prevent the storage device copy becoming too far out of date, its contents are written out at regular intervals.

20.2 The ’mount’ table (0272)

The “mount” table contained an entry for each mounted file system volume. Each entry defines the device on which the file system volume is mounted, a pointer to the buffer which stores the “super block” for the device, and an “inode” pointer. The table is referenced as follows:

iinit (6922): which is called by “main” (1615), makes an entry for the root device;
smount (6086): is a system call which makes entries for additional devices;
iget (7276): searches the “mount” table if it encounters an “inode” with the ‘IMOUNT’ flag set;
getfs (7167): searches the “mount” table to find and return a pointer to the “super block” for a particular device;
update (7201): is called periodically and searches the “mount” table to locate information which should be written from core tables into the tables maintained on the file system volumes;
sumount (6144): is a system call which deletes entries from the table.

20.3 iinit (6922)

This routine is called by “main” (1615) to initialise the “mount” table entry for the root device.

6926:

Call the “open” routine for the root device. Note that “rootdev” is defined in “conf.c” (4695);

6931:

Copy the contents of the root device “super block” into a buffer area not associated with any particular device;

6933:

The zeroeth entry in the “mount” table is assigned to the root device. Only two of the three elements are explicitly initialised. The third, the “inode” pointer, will never be referenced;

6936:

The “locks” stored in the “super block” are explicitly reset. (These locks may have been set when the “super block” was last written onto the file system volume);

6938:

The root device is mounted in a “writable” state;

6939:

The system sets its idea of the current time and date from the time recorded in the “super block”. (If the system has been stopped for an appreciable period, the computer operator will need to reset the contents of “time”.)

20.4 Mounting

From an operational view point, “mounting” a file system volume involves placing it in a suitable access device, readying the device, and then entering a command such as parameters.

    ``/etc/mount /dev/rk2 /rk2''

to the “shell”, which forks a program to perform a “mount” system call, passing pointers to the two file names as parameters.

20.5 smount (6086)

6093:

“getmdev” decodes the first argument to locate a block oriented access device;

6096:

“u.u_dirp” is reset preparatory to calling “namei” to decode the second file name. (Note that “u.u_dirp” is set by “trap” to “u.u_arg[0]” (2770);

6100:

Check that the file named by the second parameter is in a satisfactory condition, i.e. no one else is currently accessing the file, and that the file is not a special file (block or character);

6103:

Search the “mount” table looking for an empty entry (“mp-\({\gt}\)m bufp==NULL”) or an entry already made for the device. (The “mount” data structure is defined at line 0272);

6111:

“smp” should point to a suitable entry in the “mount” table;

6113:

Perform the appropriate “open” routine, with the device name and a read/write flag as arguments. (As was seen earlier, for the RK05 disk the “open” routine is a “no-op”);

6116:

Read block #1 from the device. This block is the “super block”;

6124:

Copy the “super block” into a buffer associated with “NODEV”, from the buffer associated with “d”. The second buffer will not be released again until the device is unmounted;

6130:

“ip” points to the “inode” for the second named file. This “inode” is now flagged as “IMOUNT”. The effect of this is to force “iget” (7292) to ignore the normal contents of the file, while the file system volume is mounted. (In practice, the second file is an empty file created especially for this purpose.)

20.6 Notes

1.: The “read/write” status of a mounted device depends only on the parameters provided to “smount”. No attempt is made to sense the hardware “read/write” status. Thus if a disk is readied with “write protect” on, but is not mounted “read only”, then the system will complain vigorously.
2.: The “mount” procedure does not carry out any kind of label checking on the “mounted” file system volume. This is reasonable in a situation where file system volumes are rarely rearranged. However in situations where volumes are mounted and remounted frequently, some means of verifying that the correct volume has been mounted would seem desirable. (Further, if a file system volume contains sensitive information, it may be desirable to include some form of password protection as well. There is room in the “super block” (5575) for the storage of a name and an encrypted password.)

20.7 iget (7276)

This procedure is called by “main” (1616,1618), “unlink” (3519), “ialloc” (7078) and “namei” (7534, 7664) with two parameters which together uniquely identify a file: a device, and the “inode” number of a file on the device. “iget” returns a reference to an entry in the core “inode” table.

When “iget” is called, the core “inode” table is searched first to see if an entry already exists for the file in the core “inode” table. If not, then “iget” creates one.

7285:

Search the core “inode” table ...

7286:

If an entry for the designated file already exists ...

7287:

Then if it is locked go to sleep;

7290:

Try again. (Note the whole table needs to be searched again from the beginning, because the entry may have vanished!);

7292:

If the IMOUNT flag is on ... this is an important possibility for which we will delay the discussion;

7302:

If the “IMOUNT” flag is not set, increase the “inode” reference count, set the “ILOCK” flag and return a pointer to the “inode”;

7306:

Make a note of the first empty slot in the “inode” table;

7309:

If the “inode” table is full, send a message to the operator, and take an error exit;

7314:

At this point, a new entry is to be made in the “inode” table;

7319:

Read the block which contains the file system volume “inode”. Note the use of “bread” instead of “readi”, the assumption that “inode” information begins in block #2 and the convention that valid “inode” numbers begin at one (not zero);

7326:

A read error at this point isn’t very well reported to the rest of the system;

7328:

Copy the relevant “inode” information. This code makes implicit use of the contents of the file “ino.h” (Sheet 56), which isn’t referenced explicitly anywhere.

Let us now return to unfinished business:

7292:

The “IMOUNT” flag is found to be set. This flag was set by “smount”, when a file system volume was mounted;

7293:

Search the “mount” table to find the entry which points to the curent “inode”. (Although searching this table is not a horrendous overhead, it does seem possible that a “back pointer” could be conveniently stored in in the “inode” e.g. in the “i_lastr” field. This would save both time and code space.;

7396:

Reset “dev” and “ino” to the mounted device number and the “inode” number of the root directory on the mounted file system volume. Start again.

Clearly, since “iget” is called by “namei” (7534, 7664), this technique allows the whole directory structure on the mounted file system volume to be integrated into the pre-existing directory structure. If we momentarily ignore the possible deviations of directory structures away from tree structures, we have the situation where a leaf of the existing tree is being replaced by an entire subtree.

20.8 getfs (7167)

There is little that needs to be said about this procedure in addition to the author’s comment. This procedure is called by

   access   (6754)     ialloc   (7072)
   alloc    (6961)     ifree    (7138)
   free     (7004)     iupdat   (7383)

Note the cunning use of “n1”, “n2” which are declared as character pointers i.e. as unsigned integers. This allows only one sided tests on the two variables at line 7177.

20.9 update (7201)

The function of this procedure, in its broadest terms, is to ensure that information on the file system volumes is kept up to date. The comment for this procedure (beginning on line 7190) describes the three main sub-functions, (in the reverse order!).

“update” is the whole business of the “sync” system call (3486). This may be invoked via the “sync” shell command. Alternatively there is a standard system program which runs continuously and whose only function is to call “sync” every 30 seconds. (See “UPDATE(VIII)” in the UPM.)

“update” is called by “sumount” (6150) before a file system volume is unmounted, and by “panic” (2420) as the last action of the system before activity ceases.

7207:

If another execution of “update” is under way, then just return;

7210:

Search the “mount” table;

7211:

For each mounted volume, ...

7213:

Unless the file system has not been recently modified or the “super block” is locked or the volume has been mounted “read only” ...

7217:

Update the “super block”, copy it into a buffer and write the buffer out onto the volume;

7223:

Search the “inode” table, and for each non-null entry, lock the entry and call “iupdat” to update the “inode” entry on the volume if appropriate;

7229:

Allow additional executions of “update” to commence;

7230:

“bflush” (5229) forces out any “delayed write” blocks.

20.10 sumount (6144)

This system call deletes an entry for a mounted device from the “mount” table. The purpose of this call is to ensure that traffic to and from the device is terminated properly, before the storage device is physically removed from the access device.

6154:

Search the “mount” table for the appropriate entry;

6161:

Search the “inode” table for any outstanding entries for files on the device. If any such exist, take an error exit, and do not change the “mount” table entry;

6168:

Clear the “IMOUNT” flag.

20.11 Resource Allocation

Our attention now turns to the management of the resources of an individual FSV (file system volume).

Storage blocks are allocated from the free list by “alloc” at the request of “bmap”. Storage blocks are returned to the free list by “free” at the behest of “itrunc” (which is called by “core”, “openl” and “iput”).

Entries in the FSV “inode” tables are made by “ialloc”, which is called by “maknode” and “pipe”. Entries in this table are cancelled by “ifree”, which is called by “iput”.

The “super block” for the FSV is central to the resource management procedures. The “super block” (5561) contains:

size information (total resources available);
list of up to 100 available storage blocks;
list of up to 100 available “inode” entries;
locks to control manipulation of the above lists;
flags;
current date of last update.

If the list in core of available “inode” entries for the file system volume ever becomes exhausted, then the entire table on the FSV is read and searched to rebuild the list. Conversely if the available “inode” table overflows, additional entries are simply forgotten to be rediscovered later.

A different strategy is used for the list of available storage blocks. These blocks are arranged in groups of up to one hundred blocks. The first block in each group (except the very first) is used to store the addresses of the blocks belonging to the previous group. Addresses of blocks in the last incomplete group are stored in the “super block”.

The first entry in the first list of block numbers is zero, which acts as a sentinel. Since the whole list is subject to a LIFO discipline, discovery of a block number of zero in the list signifies that the list is in fact empty.

20.12 alloc (6965)

This is called by “bmap” (6435, 6448, 6468, 6480, 6497) whenever a new storage block is needed to store part of a file.

6961:

Convert knowledge of the device name into a pointer to the “super block”;

6962:

If “s_flock” is set, the list of available blocks is currently being updated by another process;

6967:

Obtain the block number of the next available storage block;

6968:

If the last block number on the list is zero, the entire list is now empty;

6970:

“badblock” (7040) is used to check that the block number obtained from the list seems reasonable;

6971:

If the list of available blocks in the “super block” is now empty, then the block just located will contain the addresses of the next group of

6972:

Set “s_flock” to delay any other process from getting a “no space” indication before the list of available blocks in the “super block” can be replenished;

6975:

Determine the number of valid entries in the list to be copied;

6978:

Reset “s_flock”, and “wakeup” anyone waiting;

6982:

Clear the buffer so that any information recorded in the file by default will be all zeros;

6983:

Set the “modified” flag to ensure that the “super block” will be written out by “update” (7213).

20.13 itrunc (7414)

This procedure is called by “core” (4112), “openl” (5825) and “iput” (7353). In the first two cases, the contents of the “file” are about to be replaced. In the third case, the file is about to be abandoned.

7421:

If the file is a character or block special file then there is nothing to do;

7423:

Search backwards the list of block numbers stored in the “inode”;

7425:

If the file is large, then an indirect fetch is needed. (A double indirect fetch is needed for blocks numbered seven and higher.);

7427:

Reference all 257 elements of the buffer in reverse order. (Note this seems to be the only place where characters #512, #513 of the buffer area are referenced. Since they will presumably contain zero, they will contribute nothing to the calculation. Hence if “510” were substituted for “512” here, and again on line 7432, a general improvement all round would result (?));

7438:

“free” returns an individual block to the available list;

7439:

This is the end of the “for” statement commencing on line 7427. (Likewise the statement which begins at 7432 ends at 7435.);

7443:

Clear the entry in “i_addr[ ]”;

7445:

Reset size information, and flag the “inode” as “updated”.

20.14 free (7000)

This procedure is called by “itrunc” (7435, 7438, 7442) to reinsert a simple storage block into the available list for a device.

7005:

It is not clear why the “s_fmod” flag is set here as well as at the end of the procedure (line 7026). Any suggestions?

7006:

Observe the locking protocol;

7010:

If no free blocks previously existed for the device, restore the situation by setting up a one element list containing an entry for block #0. This value will subsequently be interpreted as an “end of list” sentinel;

7014:

If the available list in the “super block” is already full, it is time to write it out onto the FSV. Set “s_flock”;

7016:

Get a buffer, associated with the block now being entered in the free list;

7019:

Copy the contents of the super block list, preceded by a count of the number of valid blocks, into the buffer; write the buffer; unset the lock and “wakeup” anybody waiting,

7025:

Add the returned block to the available list.

20.15 iput (7344)

This procedure is one of the most popular in UNIX (called from nearly thirty different places) and its use will have already been frequently observed.

In essence it simply decrements the reference count for the “inode” passed as a parameter, and then calls “prele” (7882) to reset the “inode” lock and to perform any necessary “wakeup”s.

“iput” has an important side effect. If the reference count is going to be reduced to zero, then a release of resources is indicated. This may be simply the core “inode”, or both that and the file itself, if the number of links is also zero.

20.16 ifree (7134)

This procedure is called by “iput” (7355) to return a FSV “inode” to the available list maintained in the “super block”. If this list is already full (as noted above) or if the list is locked (using “s_ilock”) the information is simply discarded.

20.17 iupdat (7374)

This procedure is called by “statl” (6050), “update” (7226) and “iput” (7357) to revise a particular “inode” entry on a FSV. It does nothing if the corresponding core “inode” is not flagged (“IUPD” or “IACC”);

The “IUPD” flag may be set by one of

  unlink (3530)        bmap (6452,6467)
  chmod  (3570)        itrunc  (7448)
  chown  (3583)        maknode (7462)
  link   (5942)        namei   (7609)
  writei (6285,6318)   pipe    (7751)

The “IACC” flag may be set by one of

  readi  (6232)       maknode (7462)
  writei (6285)       pipe    (7751)

The flags are reset by “iput” (7359).

7383:

Forget it, if the FSV has been mounted as “read only”;

7386:

Read the appropriate block containing the FSV “inode” entry. As observed earlier with respect to “iget”, note the the use of “bread” instead of “readi”, the assumption that the “inode” table begins at block #2 and the convention that valid “inode” numbers begin at one;

7389:

Copy the relevant information from the core “inode”;

7391:

If appropriate, update the time of last access;

7396:

If appropriate, update the time of last modification;

7400:

Write the updated block back to the FSV.