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The file system is one of the most important parts of an operating system. The file system stores and manages user data on disk drives, and ensures that what’s read from storage is identical to what was originally written. In addition to storing user data in files, the file system also creates and manages information about files and about itself. Besides guaranteeing the integrity of all that data, file systems are also expected to be extremely reliable and have very good performance.

File systems update their structural information (called metadata) by synchronous writes. Each metadata update may require many separate writes, and if the system crashes during the write sequence, metadata may be in inconsistent state. At the next boot the filesystem check utility (called fsck) must walk through the metadata structures, examining and repairing them. This operation takes a very very long time on large filesystems. And the disk may not contain sufficient information to correct the structure. This results in misplaced or removed files. A journaling file system uses a separate area called a log or journal. Before metadata changes are actually performed, they are logged to this separate area. The operation is then performed. If the system crashes during the operation, there is enough information in the log to "replay" the log record and complete the operation. This approach does not require a full scan of the file system, yielding very quick filesystem check time on large file systems, generally a few seconds for a multiple-gigabyte file system. In addition, because all information for the pending operation is saved, no removals or lost-and-found moves are required. Disadvantage of journaling filesystems is that they are slower than other filesystems. Some journaling filesystems: BeFS, HTFS, JFS, NSS, Ext3, VxFS and XFS. 

Fortunately, a number of other Linux file systems take up where Ext2 leaves off. Indeed, Linux now offers four alternatives to Ext2:

In addition to meeting some or all of the requirements listed above, each of these alternative file systems also supports journaling, a feature certainly demanded by enterprises, but beneficial to anyone running Linux. A journaling file system can simplify restarts, reduce fragmentation, and accelerate I/O. Better yet, journaling file systems make fscks a thing of the past.

If you maintain a system of fair complexity or require high-availability, you should seriously consider a journaling file system. Let’s find out how journaling file systems work, look at the four journaling file systems available for Linux, and walk through the steps of installing one of the newer systems, JFS. Switching to a journaling file system is easier than you might think, and once you switch — well, you’ll be glad you did.

Fun with File Systems

To better appreciate the benefits of journaling file systems, let’s start by looking at how files are saved in a non-journaled file system like Ext2. To do that, it’s helpful to speak the vernacular of file systems.

A file system consists of blocks of data. The number of bytes constituting a block varies depending on the OS. The internal physical structure of a hard disk consists of cylinders. The hard disk is divided into groups of cylinders known as cylinder groups, further divided into blocks.

The file system is comprised of five main blocks (boot block, superblock, Inode block, data block,

  1. Boot block. The boot block is part of the disk label that contains a loader used to boot the operating system.
  2. Super block. All partitions within the Unix filing system usually contain a special block called the super block. The super block contains the basic information about the entire file system. It stores the following details about the file system:
  3. Inode block. Inode is a kernel structure that contains a pointer to the disk blocks that store data. This pointer points to information such as file type, permission type, owner and group information, file size, file modification time, and so on. Note that the inode does not contain the filename as part of the information. The filename is listed in a directory that contains a list of filenames and related inodes associated with the file. When a user attempts to access a given file by name, the name is looked up in the directory where the corresponding inode is found. Inode stores the following information about every file:

    Each inode has a unique number associated with it, called the inode number. The -li option of the ls command displays the inode number of a file:

    # ls -li

    When a user creates a file in the directory or modifies it, the following events occur:


  4. Data block

    The data block is the storage unit of data in the Solaris file system. The default size of a data block in the Solaris file system is 8192 bytes. After a block is full, the file is allotted another block. The addresses of these blocks are stored as an array in the Inode.

    The first 12 pointers in the array are direct addresses of the file; that is, they point to the first 12 data blocks where the file contents are stored. If the file grows larger than these 12 blocks, then a 13th block is added, which does not contain data. This block, called an indirect block, contains pointers to the addresses of the next set of direct blocks.

    If the file grows still larger, then a 14th block is added, which contains pointers to the addresses of a set of indirect blocks. This block is called the double indirect block. If the file grows still larger, then a 15th block is added, which contains pointers to the addresses of a set of double indirect blocks. This block is called the triple indirect block.

  5. Vnodes. A Virtual Node or vnode is a data structure that represents an open file, directory, or device that appears in the file system namespace. A vnode does not render the physical file system it implements. The vnode interface allows high-level operating system modules to perform uniform operations on vnodes.


Hard and soft links are a great features of Unix. It is a reference in a directory to a file stored in another directory. In case of soft links it can be a reference to a directory. There might be multiple links to a file. Links eliminate redundancy because you do not need to store multiple copies of a file.

Links are of two types: hard and soft (also known as symbolic).

To create a symbolic link, you must use the -s option with the ln command. Files that are soft linked contain an l symbol at the first bit of the access permission bits displayed by the ls -l command, whereas those that are hard linked do not contain the l symbol. A directory is symbolically linked to a file. However, it cannot be hard linked.

It is obvious that no file exists with a link count less than one.  Relative pathnames . or .. are nothing but links for the current directory and its parent directory.  These are present in every directory: any directory stores the two links ., .. and the Inode numbers of the files. They can be listed by the ls -lia option. A directory must have a minimum of two links. The number of links increases as the number of sub-directories increase. Whenever you issue a command to list the file attributes, it refers to the Inode block with the Inode number and the corresponding data is retrieved.

Solaris File Systems and Their Functions

Each file system used in Solaris is intended for a specific purpose.

The root file system is at the top of an inverted tree structure. It is the first file system that the kernel mounts during booting. It contains the kernel and device drivers. The / directory is also called the mount pointdirectory of the file system. All references in the file system are relative to this directory. The entire file system structure is attached to the main system tree at the root directory during the process of mounting, and hence the name. During the creation of the file system, a lost + found directory is created within the mount point directory. This directory is used to dump into the file system any unredeemed files that were found during the customary file system check, which you do with the fsck command.

/ (root)

The directory located at the top of the Unix file system. It is represented by the "/" (forward slash) character.

/usr Contains commands and programs for system-level usage and administration.

/var Contains system log files and spooling files, which grow in size with system usage.

/home Contains user home directories.

/opt Contains optional third-party software and applications.

/tmp Contains temporary files, which are cleared each time the system is booted.

/proc Contains information about all active processes.

You create file systems with the newfs command. The newfs command accepts only logical raw device names. The syntax is as follows:

newfs [ -v ] [ mkfs-options ] raw-special-device

For example, to create a file system on the disk slice c0t3d0s4, the following command is used:

# newfs -v /dev/rdsk/c0t3d0s4

The -v option prints the actions in verbose mode. The newfs command calls the mkfs command to create a file system. You can invoke the mkfs command directly by specifying a -F option followed by the type of file system.

Mounting File Systems

Mounting file systems is the next logical step to creating file systems. Mounting refers to naming the file system and attaching it to the inverted tree structure. This enables access from any point in the structure. A file system can be mounted during booting, manually from the command line, or automatically if you have enabled the automount feature.

With remote file systems, the server shares the file system over the network and the client mounts it.

The / and /usr file systems, as mentioned earlier, are mounted during booting. To mount a file system, attach it to a directory anywhere in the main inverted tree structure. This directory is known as the mount point. The syntax of the mount command is as follows:

# mount <logical block device name>   <mount point>

The following steps mount a file system c0t2d0s7 on the /export/home directory:

# mkdir /export/home
# mount  /dev/dsk/c0t2d0s7 /export/home

You can verify the mounting by using the mount command, which lists all the mounted file systems.

Note:  If the mount point directory has any content prior to the mounting operation, it is hidden and remains inaccessible until the file system is unmounted.

Data is stored and retrieved from the physical disk where the file system is mounted.

Although there are no defined specifications for creating the file systems on the physical disk, slices are usually allocated as following:

    0.  Root or /— Files and directories of the OS.

  1. Swap— Virtual memory space.
  2. Refers to the entire disk.
  3. /export— Different OS versions.
  4. /export/swap— Unused. Left to user's choice.
  5. /opt— Application software added to a system.
  6. /usr— OS commands by users.
  7. /home— Files created by users.

The slices shown above are all allocated on a single single disk. However, there is no restriction that all file systems need to be located on a single disk. They can also span across multiple disks. Slice 2 refers to the entire disk. Hence, if you want to allocate an entire disk for a file system, you can do so by creating it on slice 2. The mount command supports a variety of useful options.

Option Description
-o largefiles Files larger than 2GB are supported in the file system.
-o nolargefiles Does not mount file systems with files larger than 2GB.
-o rw File system is mounted with read and write permissions.
-o ro File system is mounted with read-only permission.
-o bg Repeats mount attempts in the background. Used with non-critical file systems.
-o fg Repeats mount attempts in the foreground. Used with critical file systems.
-p Prints the list of mounted file systems in /etc/vfstab format.
-m Mounts without making an entry in /etc/mnt /etc/tab file.
-O Performs an Overlay mount. Mounts over an existing mount point.

The mountall command mounts all file systems that have the mount at boot field in the /etc/vfstab file set to yes. It can also be used anytime after booting.

Unmounting File Systems

A file system can be unmounted with the umount command. The following is the syntax for umount:

umount <mount-point or logical block device name >
File systems cannot be unmounted when they are in use or when the umount command is issued from any subdirectory within the file system mount point.
Note: A file system can be unmounted forcibly if you use the -f option of the umount command. Please refer to the man page to learn about the use of these options.

The umountall command is used to unmount a group of file systems. The umountall command unmounts all file systems in the /etc/mnttab file except the /, /usr, /var, and /proc file systems. If you want to unmount all the file systems from a specified host, use the -h option. If you want to unmount all the file systems mounted from remote hosts, use the -r option.

/etc/vfstab File

The /etc/vfstab (Virtual File System Table) file plays a very important role in system operations. This file contains one record for every device that has to be automatically mounted when the system enters run level 2.

Column Name Description
device to mount The logical block name of the device to be mounted. It can also be a remote resource name for NFS.
device to fsck The logical raw device name to be subjected to the fsck check during booting. It is not applicable for read-only file systems, such as High Sierra File System (HSFS) and network File systems such as NFS.
Mount point The mount point directory.
FS type The type of the file system.
fsck pass The number used by fsck to decide whether the file system is to be checked.

0— File system is not checked.

1— File system is checked sequentially.

2— File system is checked simultaneously along with other file systems where this field is set to 2.

Mount at boot The file system to be mounted by the mount all command at boot time is determined by this field. The options are either yes or no.
Mount options The mount options to be supported by the mount command while the particular file system is mounted.

Note the no values in this field for the root, /usr, and /var file systems. These are mounted by default. The fd field refers to the floppy disk and the swap field refers to the tmpfs in the /tmp directory.

A sample vfstab file looks like:

#device         device          mount           FS      fsck    mount   mount
#to mount       to fsck         point           type    pass   at boot options
fd      -       /dev/fd fd      -       no      -
/proc   -       /proc   proc    -       no      -
/dev/dsk/c0t0d0s4       -       -       swap    -       no      -
/dev/dsk/c0t0d0s0       /dev/rdsk/c0t0d0s0      /       ufs     1     no
/dev/dsk/c0t0d0s6       /dev/rdsk/c0t0d0s6      /usr    ufs     1     no
/dev/dsk/c0t0d0s3       /dev/rdsk/c0t0d0s3      /var    ufs     1     no
/dev/dsk/c0t0d0s7       /dev/rdsk/c0t0d0s7      /export/home    ufs   2
yes     -
/dev/dsk/c0t0d0s5       /dev/rdsk/c0t0d0s5      /opt    ufs     2     yes
/dev/dsk/c0t0d0s1       /dev/rdsk/c0t0d0s1      /usr/openwin    ufs   2 yes  -
swap    -   /tmp    tmpfs   -       yes     -

Finding Information About the Mounted File Systems

The /etc/mnttab file comprises a table that defines which partitions and/or disks are currently mounted by the system.

The /etc/mnttab file contains the following details about each mounted file system:

A sample mnttab file:

/dev/dsk/c0t0d0s0       /       ufs     rw,intr,largefiles,xattr,onerror=panic,s
uid,dev=2200000 1014366934
/dev/dsk/c0t0d0s6       /usr    ufs     rw,intr,largefiles,xattr,onerror=panic,s
uid,dev=2200006 1014366934
/proc   /proc   proc    dev=4300000     1014366933
mnttab  /etc/mnttab     mntfs   dev=43c0000     1014366933
fd      /dev/fd fd      rw,suid,dev=4400000     1014366935
/dev/dsk/c0t0d0s3       /var    ufs     rw,intr,largefiles,xattr,onerror=panic,s
uid,dev=2200003 1014366937
swap    /var/run        tmpfs   xattr,dev=1     1014366937
swap    /tmp    tmpfs   xattr,dev=2     1014366939
/dev/dsk/c0t0d0s5       /opt    ufs     rw,intr,largefiles,xattr,onerror=panic,s
uid,dev=2200005 1014366939
/dev/dsk/c0t0d0s7       /export/home    ufs     rw,intr,largefiles,xattr,onerror
=panic,suid,dev=2200007 1014366939
/dev/dsk/c0t0d0s1       /usr/openwin    ufs     rw,intr,largefiles,xattr,onerror
=panic,suid,dev=2200001 1014366939
-hosts  /net    autofs  indirect,nosuid,ignore,nobrowse,dev=4580001     10143669
auto_home       /home   autofs  indirect,ignore,nobrowse,dev=4580002    10143669
-xfn    /xfn    autofs  indirect,ignore,dev=4580003     1014366944
sun:vold(pid295)        /vol    nfs     ignore,dev=4540001      1014366950

Restricting the File Size

Some applications and processes create temporary files that occupy a lot of hard disk space. As a result, it is necessary to impose a restriction on the size of the files that are created.

Solaris provides tools to control the storage. They are:

ulimit Command

The ulimit command is a built-in shell command, which displays the current file size limit. The default value for the maximum file size, set inside the kernel, is 1500 blocks. The following syntax displays the current limit:

$ ulimit -a
time(seconds) unlimited
file(blocks) unlimited
data(kbytes) unlimited
stack(kbytes) 8192
coredump(blocks) unlimited
nofiles(descriptors) 256
memory(kbytes) unlimited

If the limit is not set, it reports as unlimited.

The system administrator and the individual users change this value to set the file size at the system level and at the user level, respectively. The following is the syntax of the ulimit command:

ulimit <value>

For example, the following syntax sets the file size limit to 1600 blocks:

# ulimit  1600

# ulimit -a
time(seconds) unlimited
file(blocks) 1600
data(kbytes) unlimited
stack(kbytes) 8192
coredump(blocks) unlimited
nofiles(descriptors) 256
memory(kbytes) unlimited

The file size can be limited at the system level or the user level. To set it at the system level, change the value of the ulimit variable in the /etc/profile file. To set it at the user level, change the value in the .profile file present in the user's home directory. The user-level setting always takes precedence over the system-level setting. It is the user's profile file that sets the working environment.

Note: The ulimit values set at the user level and system level cannot exceed the default ulimit value set in the kernel.

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Old News ;-)

[Feb 04, 2017] Restoring deleted /tmp folder

Jan 13, 2015 |

As my journey continues with Linux and Unix shell, I made a few mistakes. I accidentally deleted /tmp folder. To restore it all you have to do is:

mkdir /tmp
chmod 1777 /tmp
chown root:root /tmp
ls -ld /tmp
mkdir /tmp chmod 1777 /tmp chown root:root /tmp ls -ld /tmp 

The Evolution of Stupidity: File Systems

The storage industry continues to make the same mistakes over and over again, and enterprises continue to take vendors' bold statements as facts. Previously, we introduced our two-part series, "The Evolution of Stupidity," explaining how issues seemingly resolved more than 20 years ago are again rearing their heads. Clearly, the more things change, the more they stay the same.

This time I ask, why do we continue to believe that the current evolutionary file system path will meet our needs today and in the future and cost nothing? Let's go back and review a bit of history for free and non-free systems file systems.

Time Machine -- Back to the Early 1980s

My experiences go back only to the early 1980s, but we have repeated history a few times since then. Why can we not seem to remember history, learn from it or even learn about it? It never ceases to amaze me. I talk to younger people, and more often than not, they say that they do not want to do hear about history, just about the presentation, and how they are going to make the future better. I coined a saying (at least I think I coined it) in the late 1990s: There are no new engineering problems, just new engineers solving old problems. I said this when I was helping someone develop a new file system using technology and ideas I had helped optimize the design around 10 years earlier.

In the mid-1980s, most of the open system file systems came as part of a standard Unix release from USL. A few vendors, such as Cray and Amdahl, wrote their own file systems. These vendors generally did so because the standard UNIX file did not meet the requirements of the day. UFS on Solaris came from another operating system, which was written in the 1960s, called Multics . That brings us to the late 1980s, and by this time, we had a number of high-performance file systems from companies such as Convex, MultiFlow and Thinking Machines. Everyone who had larger systems had their own file system, and everyone was trying to address many, if not all, of the same issues. They were in my opinion the scalability of:

  1. Metadata performance
  2. Recovery performance
  3. Small block performance
  4. Large block performance
  5. Storage management

The keyword here is scalability. Remember, during this time disk drive density was growing very rapidly and performance was scaling far better than it is today. Some of the vendors began the process of looking at parallel systems and some began charging for file systems that were once free. Does any of this sound like what I said in a recent blog post, "It's like deja-vu, all over again" (Yogi Berra)? But since this article is about stupidity, let's also remember the quote from another Yogi, Yogi Bear the cartoon character, "I'm smarter than the average bear!" and ask the question: Is the industry any smarter?

Around 1990, Veritas released VxFS, the first commercial UNIX file system. This file system tried to address all of the bullets points above except storage management, and Veritas added that later with VxVM. VxFS was revolutionary for commercial UNIX implementations at the time. Most of the major vendors used this product in some fashion, either supporting it or OEMing. Soon Veritas added things like the DB edition, which removed some of the POSIX-required write lock restrictions.

While Veritas was taking over the commercial world in the 1990s and making money on the file system, Silicon Graphics (SGI) decided to write its own file system, called XFS. It was released in the mid-1990s. It was later open sourced and had similar some characteristic to VxFS (imagine that), given that some of the developers were the same people. By the late 1990s and early 2000s, a number of vendors had shared file systems, but you had to pay for most of them in the HPC community. Most were implemented with a single metadata server and clients. Meanwhile, a smaller number of vendors were trying to solve large shared data problems problems with a shared name space and implementation of distributed allocation of space.

Guess what? None of these file systems were free, and all of them were trying to resolve the list of the five areas noted above. From about 2004 until Sun Microsystems purchased CFS in 2007, there was an exception to the free parallel file system-- Lustre. But "free" is relative because for much of that time significant funding was coming from the U.S. government. It was not long after the funding ran out that Sun Microsystems purchased the company that developed the Lustre file system and hoped to monetize the company purchase cost by developing hardware around the Lustre file system.

Related Articles

At the same time, on the commercial front, the move to Linux was in full swing. Enter the XFS file system, which came with many standard Linux distributions and met many requirements. Appliance-based storage from the NAS vendors also met many of the requirements for performance and was far easier to management than provisioning file systems from crop of file system vendors selling file systems.

Now you have everyone moving to free file systems, not from vendors like in the 1980s but from the Linux distribution or from the NAS appliances vendors. Storage is purchased with a built-in file system.

This is all well and good, but now I am seeing the beginnings of change back to the early 1990s. Remember the saying that railroad executives in the 1920s and 1930s did not realize they were in the transportation business? Rather, they saw themselves as being only in the railroad business and thus did not embrace the airline industry. Similarly, NAS vendors do not seem to realize they are in the scalable storage business, and large shared file system vendors are now building appliances to better address many of the five bullets above.

Why Are We Going Around in Circles?

It seems to me that we are going around in circles. The 1980s are much like the early 2000s in the file system world, and the early 1990s are like the mid-2000s. The mid-1990s are similar to what we are going into again. The same is likely true for other areas of computing, as I have shown for storage in the previous article If we all thought about it, that could be said for computational design with scalar processors, vector process, GPUs and FPGAs today and yesteryear.

So everything is new every 20 years or so, and the problem solutions are not really that different. Why? Is it because no one remembers the past? Is it because everyone thinks they are smarter than their manager was when he or she was doing the work 20 years ago? Or is it something far different, like the market mimics other cycles in life like fashion, food preparation and myriad other things.

Almost 20 years ago, some friends of mine at Cray Research had the idea to separate file system metadata and data on different storage technologies, as data and metadata had different access patterns. Off and on file systems over the past 20 years have done this, but the concept has never really caught on as a must-have for file systems. I am now hearing rumblings that lots of people are talking about doing this with xyz file system. Was this NIH? I think in some cases, yes. The more I think about it, there is not a single answer to explain what happens, and if I did figure it out, I will be playing the futures market rather than doing what I am doing. We all need to learn from the past if we are going to break this cycle and make dramatic changes to technology.

POSIX is now about 20 years old since the last real changes were made. I am now hearing that POSIX limitations from a number of circles are limiting the five factors. If we change POSIX to support parallel I/O, I hope we look beyond today and think to the future.

Henry Newman is CEO and CTO of Instrumental Inc. and has worked in HPC and large storage environments for 29 years. The outspoken Mr. Newman initially went to school to become a diplomat, but was firmly told during his first year that he might be better suited for a career that didn't require diplomatic skills. Diplomacy's loss was HPC's gain.

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Last modified: October 30, 2017