Filesystem Management Subsystem
In discussing "Kernel Basics and Configuration" a very important topic, filesystems, must be considered. This discussion shall deal with the basic structural method of long-term storage of system and user data. Filesystems and the parameters that are used to create them have a direct impact on performance, system resource utilization, and kernel efficiency dealing with Input/Output (I/O).
Filesystem Types
There are several important filesystem types that are supported by different operating systems (OS), many of which are not used for implementation at this time. The reasons they are not used vary from being inefficient to just being outdated. However, many operating systems still support their filesystem structure so that compatibility doesn't become an issue for
portability.
This support of other filesystem structures plays a large role in allowing companies to move between OS and computer types with little impact to their applications.
The following is a list of filesystem types that are supported by specific operating systems. The list will only cover local, network, and CD-ROM filesystems.
Local Filesystem | NFS* | CD-ROM | |
Solaris | ufs | yes | bsfs |
SunOS | 4.2 | yes | bsfs |
SCO | EAFS | yes | HS |
IRIX | efs | yes | iso9660 |
Digital | ufs | yes | cdfs |
HP-UX | bfs | yes | cdfs |
AIX | jfs | yes | cdrfs |
Linux | ext2 | yes | iso9660 |
Note: NFS stands for Networked FileSystem
Hardware Architecture
Since filesystems are stored on disk, the systems administrator should look at basic disk hardware architecture before proceeding with specifics of filesystems.
A disk is physically divided into tracks, sectors, and blocks. A good representation of a sector would be a piece of pie removed form the pie pan. Therefore, as with a pie, a disk is composed of several sectors (see Figure 19.3). Tracks are concentric rings going from the outside perimeter to the center of the disk, with each track becoming smaller as it approaches the center of the disk. Tracks on a disk are concentric, therefore they never touch each other. The area of the track that lies between the edges of the sector is termed a block, and the block is the area where data is stored. Disk devices typically use a block mode accessing scheme when transferring data between the file management subsystem and the I/O subsystem. The block size is usually 512- or 1024-byte fixed-length blocks, depending upon the scheme used by the operating system. A programmer may access files using either block or character device files.
Figure 19.3.
Diagram of a single platter from a hard drive showing disk geometry.
Diagram of a single platter from a hard drive showing disk geometry.
You now have a basic understanding of the terms tracks, sectors, and blocks as they apply to a single platter disk drive. But most disk today are composed of several platters with each platter having its own read/write head. With this in mind, we have a new term: cylinder (see Figure 19.4). Let's make the assumption that we have a disk drive that has six platters so, logically, it must have six read/write heads. When read/write head 1 is on track 10 of platter 1, then heads 2 through 6 are on track 10 of their respective platters. You now have a cylinder. A cylinder is collectively the same track on each platter of a multi-platter disk.
Figure 19.4.
Diagram showing multiple platters of a single disk drive.
Diagram showing multiple platters of a single disk drive.
Filesystem Concepts and Format
The term filesystem has two connotations. The first is the complete hierarchical filesystem tree. The second is the collection place on disk device(s) for files. Visualize the filesystem as consisting of a single node at the highest level (ROOT) and all other nodes descending from the root node in a tree-like fashion (see Figure 19.5) . The second meaning will be used for this discussion, and Hewlett Packard's High-performance Filesystem will be used for technical reference purposes.
Figure 19.5.
Diagram of a UNIX hierarchical filesystem.
Diagram of a UNIX hierarchical filesystem.
The superblock is the key to maintaining the filesystem. It's an 8 KB block of disk space that maintains the current status of the
filesystem. Because of its importance, a copy is maintained in memory and at each cylinder group within the filesystem. The copy in main memory is updated as events transpire. The update daemon is the actual process that calls on the kernel to flush the cached superblocks, modified inodes, and cached data blocks to disk. The superblock maintains the following static and dynamic information about the filesystem. An asterisk will denote dynamically maintained information.
Filesystem size
Number of Inodes
Location of free space
Number of cylinder groups
Fragment size and number
Block size and number
Location of superblocks, cylinder groups, inodes, and data blocks
Total number of free data blocks
Total number of free inodes
Filesystem status flag (clean flag)
As you can see from the listed information, the superblock maintains the integrity of the filesystem and all associated pertinent information. To prevent catastrophic events, the OS stores copies of the superblock in cylinder groups. The locations of these alternate superblocks may be found in /etc/sbtab. When system administrators are using fsck -b to recover from an alternate superblock, they will be required to give the location of that alternate block. Again, the only place to find that information is in /etc/sbtab. As a qualification to that statement, there is always an alternate superblock at block sixteen.
Cylinder groups are adjacent groups of cylinders, 16 cylinders by default, that have their own set of inodes and free space mapping. This is done to improve performance and reduce disk latency. Disk latency is the time between when the disk is read and the I/O subsystem can
transfer the data. Some factors that affect disk latency are rotational speed, seek time, and the interleave factor. This concept also associates the inodes and data blocks in closer proximity.
NOTE: The interleave factor is the value that determines the order in which sectors on a disk drive are accessed.
The layout of the cylinder group is:
Boot block
Primary superblock
Redundant superblock
Cylinder group information
Inode table
Data blocks
The boot block and the primary superblock will only be there if this is the first cylinder group; otherwise, it may be filled with data.
Inodes are fixed-length entries that vary in their length according to the OS implemented. SVR4
implementation is 128 bytes for a UFS inode and 64 bytes for an S5 inode. The inode maintains all of the pertinent information about the file except for the filename and the data. The information maintained by the inode is as follows:
File permissions or mode
Type of file
Number of hard links
Current owner
Group associated to the file
Actual file size in bytes
Time Stamps
Time/Date file last changed
Time/Date file last accessed
Time/Date last inode modification
Single indirect block pointer
Double indirect block pointer
Triple indirect block pointer
There are 15 slots in the inode structure for disk address or pointers(see Figure 19.6). Twelve of the slots are for direct block addressing. A direct address can
either point to a complete block or to a fragment of that block. The block and fragment sizes we are discussing are configurable parameters that are set at filesystem creation. They cannot be altered unless the filesystem is removed and re-created with the new parameters.
Figure 19.6.
Diagram of an Inode Structure of a UNIX filesystem
Diagram of an Inode Structure of a UNIX filesystem
Listing of a typical AIX Root directory using ls -ali, to indicate the inode numbers for each file entry in the directory.
inode | Permissions | ln | Owner | Group | Size | Access Date | Filename |
2 | drwxr-xr-x | 23 | bin | bin | 1024 | Apr 27 15:53 | . (dot) |
2 | drwxr-xr-x | 23 | bin | bin | 1024 | Apr 27 15:53 | .. (dot, dot) |
765 | -rw-r--r-- | 1 | root | system | 259 | Apr 08 08:34 | Guidefaults |
1257 | -rw------- | 1 | root | system | 156 | Apr 27 11:01 | .Xauthority |
2061 | drwxr-xr-x | 11 | root | system | 512 | Apr 27 11:01 | .dt |
591 | -rwxr-xr-x | 1 | root | system | 3970 | Apr 08 08:38 | .dtprofile |
6151 | drwx------ | 3 | root | system | 512 | Apr 17 13:42 | .netscape |
593 | -rw------- | 1 | root | system | 1904 | Apr 11 08:12 | .old_sh_history |
1011 | -rwxr----- | 1 | 7 | system | 254 | Apr 10 11:15 | .profile |
1007 | -rw------- | 1 | root | system | 3444 | Apr 27 15:53 | .sh_history |
1009 | -rw-r--r-- | 1 | root | system | 30 | Apr 14 10:35 | .showcase |
2069 | drwxr-xr-x | 2 | root | system | 512 | Apr 08 08:54 | TT_DB |
2058 | drwxr-xr-x | 3 | root | system | 512 | Apr 11 11:21 | admin |
109 | lrwxrwxrwx | 1 | bin | bin | 8 | Apr 01 05:27 | bin ->/usr/bin |
23 | drwxrwxr-x | 4 | root | system | 2048 | Apr 27 14:37 | dev |
24 | drwxr-xr-x | 12 | root | system | 6144 | Apr 27 11:29 | etc |
2 | drwxr-xr-x | 5 | bin | bin | 512 | Apr 02 01:52 | home |
8195 | drwxr-xr-x | 2 | root | system | 512 | Apr 25 13:08 | httpd |
586 | lrwxrwxrwx | 1 | bin | bin | 20 | Apr 02 01:57 | launch_demo -> |
22 | lrwxrwxrwx | 1 | bin | bin | 8 | Apr 01 05:27 | lib ->/usr/lib |
16 | drwx------ | 2 | root | system | 512 | Apr 01 05:27 | lost+found |
100 | drwxr-xr-x | 26 | bin | bin | 1024 | Apr 11 15:23 | lpp |
101 | drwxr-xr-x | 2 | bin | bin | 512 | Apr 01 05:27 | mnt |
4096 | drwxr-xr-x | 2 | root | system | 512 | Apr 11 14:57 | mnt10032 |
4097 | drwxr-xr-x | 2 | root | system | 512 | Apr 14 10:31 | mnt10086 |
1251 | -rw-rw-rw- | 1 | root | system | 3192 | Apr 15 14:12 | nv6000.log |
102 | drwxr-xr-x | 2 | root | system | 512 | Apr 02 01:54 | opt |
103 | drwxr-xr-x | 3 | bin | bin | 512 | Apr 11 15:23 | sbin |
1252 | -rw-r--r-- | 1 | root | system | 39265 | Apr 27 13:29 | smit.log |
1253 | -rw-r--r-- | 1 | root | system | 5578 | Apr 27 13:24 | smit.script |
271 | drwxrwxr-x | 2 | root | system | 512 | Apr 01 05:37 | tftpboot |
2 | drwxrwxrwt | 9 | bin | bin | 1536 | Apr 27 15:47 | tmp |
99 | lrwxrwxrwx | 1 | bin | bin | 5 | Apr 01 05:27 | u ->/home |
192 | lrwxrwxrwx | 1 | root | system | 21 | Apr 01 05:30 | unix ->/usr/lib/boot/unix_up |
2 | drwxr-xr-x | 26 | bin | bin | 512 | Apr 25 13:19 | usr |
2 | drwxr-xr-x | 14 | bin | bin | 512 | Apr 01 06:03 | var |
764 | -rw-rw-rw- | 1 | root | system | 3074 | Apr 08 08:33 | vim.log |
2 | drwxr-xr-x | 12 | bin | bin | 2048 | Apr 08 08:21 | welcome |
Single indirect addressing (slot 13) points to a block of four-byte pointers that point to data blocks. If the block that is pointed to by the single indirect method is 4 KB in size, it would contain 1024 four-byte pointers, and if it were 8 KB in size, it would contain 2048 four-byte pointers to data blocks. The double indirect
block pointer is located in slot 14, and slot 15 maintains the triple indirect block pointer.
In the "Filesystem Concepts and Format" section, the initial discussion covered basic concepts of superblocks, alternate superblocks, cylinder groups, inodes, and direct and indirect addressing of data blocks. Further reading into these subjects is a must for all systems administrators, especially the new and inexperienced.
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