Operating Systems

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4.3.6 Journaling File Systems

Many seemingly simple I/O operations are actually composed of sub-actions. For example, deleting a file on an i-node based system (really this means deleting the last link to the i-node) requires removing the entry from the directory, placing the i-node on the free list, and placing the file blocks on the free list.

What happens if the system crashes during a delete and some, but not all three, of the above actions occur?

A journaling file system prevents the problems by using an idea from database theory, namely transaction logs. To ensure that the multiple sub-actions are all performed the larger I/O operation (delete in the example) is broken into 3 steps.

  1. Write a log stating what has to be done and ensure it is written to disk.
  2. Start doing the sub-actions.
  3. When all sub-actions complete, mark the log entry as complete (and eventually erase it)..

After a crash, the log (called a journal) is examined and if there are pending sub-actions, they are done before the system is made available to users.

Since sub-actions may be repeated (once before the crash, and once after), it is required that they all be idempotent (applying the action twice is the same as applying it once).

Some history.

4.3.7 Virtual File Systems


A single operating system needs to support a variety of file systems. The software support for each file system would have to handle the various I/O system calls defined.

Not surprisingly the various file systems often have a great deal in common and large parts of the implementations would be essentially the same. Thus for software engineering reasons one would like to abstract out the common part.

This was done by Sun Microsystems when they introduced NFS the Network File System for Unix and by now most unix-like operating systems have adopted this idea. The common code is called the VFS layer and is illustrated on the right.

I consider the idea of VFS as good software engineering, rather than OS design. The details are naturally OS specific.

4.4 File System Management and Optimization

Since I/O operations can dominate the time required for complete user processes, considerable effort has been expended to improve the performance of these operations.

4.4.1 Disk Space Management

All general purpose file systems use a (non-demand) paging algorithm for file storage (read-only systems, which often use contiguous allocation, are the major exception). Files are broken into fixed size pieces, called blocks that can be scattered over the disk. Note that although this is paging, it is not called paging (and may not have an explicit page table).

Actually, it is more complicated since various optimizations are performed to try to have consecutive blocks of a single file stored consecutively on the disk. This is discussed below.

Note that all the blocks of the file are stored on the disk, i.e., it is not demand paging.

One can imagine systems that do utilize demand-paging-like algorithms for disk block storage. In such a system only some of the file blocks would be stored on disk with the rest on tertiary storage (some kind of tape). Perhaps NASA does this with their huge datasets.

Choice of Block Size

We discussed a similar question before when studying page size.

There are two conflicting goals, performance and efficiency.

  1. We will learn next chapter that large disk transfers achieve much higher total bandwidth than small transfers due to the comparatively large startup time required before any bytes are transferred. This favors a large block size.
  2. Internal fragmentation favors a small block size. This is especially true for small files, which would use only a tiny fraction of a large block and thus waste much more than the 1/2 block average internal fragmentation found for random sizes.

For some systems, the vast majority of the space used is consumed by the very largest files. For example, it would be easy to have a few hundred gigabytes of video. In that case the space efficiency of small files is largely irrelevant since most of the disk space is used by very large files.

Typical block sizes today are 4-8KB.

Keeping Track of Free Blocks

There are basically two possibilities, a bit map and a linked list.

Free Block Bitmap

A region of kernel memory is dedicated to keeping track of the free blocks. One bit is assigned to each block of the file system. The bit is 1 if the block is free.

If the block size is 8KB the bitmap uses 1 bit for every 64 kilobits of disk space. Thus a 64GB disk would require 1MB of RAM to hold its bitmap.

One can break the bitmap into (fixed size) pieces and apply demand paging. This saves RAM at the cost of increased I/O.

Linked List of Free Blocks

A naive implementation would simply link the free blocks together and just keep a pointer to the head of the list. This simple scheme has poor performance since it requires an extra I/O for every acquisition or return of a free block.

In the naive scheme a free disk block contains just one pointer; whereas it could hold around a thousand of them. The improved scheme, shown on the right, has only a small number of the blocks on the list. Those blocks point not only to the next block on the list, but also to many other free blocks that are not directly on the list.

As a result only one in about 1000 requests for a free block requires an extra I/O, a great improvement.

Unfortunately, a bad case still remains. Assume the head block on the list is exhausted, i.e. points only to the next block on the list. A request for a free block will receive this block, and the next one on the list is brought it. It is full of pointers to free blocks not on the list (so far so good).

If a free block is now returned we repeat the process and get back to the in-memory block being exhausted. This can repeat forever, with one extra I/O per request.

Tanenbaum shows an improvement where you try to keep the one in-memory free block half full of pointers. Similar considerations apply when splitting and coalescing nodes in a B-tree.

Disk Quotas

Two limits can be placed on disk blocks owned by a given user, the so called soft and hard limits. A user is never permitted to exceed the hard limit. This limitation is enforced by having system calls such as write return failure if the user is already at the hard limit.

A user is permitted to exceed the soft limit during a login session provided it is corrected prior to logout. This limitation is enforced by forbidding logins (or issuing a warning) if the user is above the soft limit.

Often files on directories such as /tmp are not counted towards either limit since the system is permitted to deleted these files when needed.

4.4.2 File System Backups (a.k.a. Dumps)

A physical backup simply copies every block in order onto a tape (or other backup media). It is simple and useful for disaster protection, but not useful for retrieving individual files.

We will study logical backups, i.e., dumps that are file and directory based not simply block based.

Tanenbaum describes the (four phase) unix dump algorithm.

All modern systems support full and incremental dumps.

An interesting problem is that tape densities are increasing slower than disk densities so an ever larger number of tapes are needed to dump a full disk. This has lead to disk-to-disk dumps; another possibility is to utilize raid, which we study next chapter.

4.4.3 File System Consistency

Modern systems have utility programs that check the consistency of a file system. A different utility is needed for each file system type in the system, but a wrapper program is often created so that the user is unaware of the different utilities.

The unix utility is called fsck (file system check) and the window utility is called chkdsk (check disk).

Bad blocks on disks

Not so much of a problem now. Disks are more reliable and, more importantly, disks and disk controllers take care most bad blocks themselves.

4.4.4 File System Performance


Demand paging again!

Demand paging is a form of caching: Conceptually, the process resides on disk (the big and slow medium) and only a portion of the process (hopefully a small portion that is heavily access) resides in memory (the small and fast medium).

The same idea can be applied to files. The file resides on disk but a portion is kept in memory. The area in memory used to for those file blocks is called the buffer cache or block cache.

Some form of LRU replacement is used.

The buffer cache is clearly good and simple for reads.

What about writes?

Homework: 27.

Block Read Ahead

When the access pattern looks sequential read ahead is employed. This means that after completing a read() request for block n of a file, the system guesses that a read() request for block n+1 will shortly be issued and hence automatically fetches block n+1.

Reducing Disk Arm Motion

The idea is to try to place near each other blocks that are likely to be accessed sequentially.

  1. If the system uses a bitmap for the free list, it can allocate a new block for a file close to the previous block (guessing that the file will be accessed sequentially).
  2. The system can perform allocations in super-blocks, consisting of several contiguous blocks.
    • The block cache and I/O requests are still in blocks not super-blocks.
    • If the file is accessed sequentially, consecutive blocks of a super-block will be accessed in sequence and these are contiguous on the disk.
  3. For a unix-like file system, the i-nodes can be placed in the middle of the disk, instead of at one end, to reduce the seek time needed to access an i-node followed by a block of the file.
  4. The system can logically divide the disk into cylinder groups, each of which is a consecutive group of cylinders.
    • Each cylinder group has its own free list and, for a unix-like file system, its own space for i-nodes.
    • If possible, the blocks for a file are allocated in the same cylinder group as the i-node.
    • This reduces seek time if consecutive accesses are for the same file.

4.4.5 Defragmenting Disks

If clustering is not done, files can become spread out all over the disk and a utility (defrag on windows) should be run to make the files contiguous on the disk.

4.5 Example File Systems

4.5.A The CP/M File System

CP/M was a very early and simple OS. It ran on primitive hardware with very little ram and disk space. CP/M had only one directory in the entire system. The directory entry for a file contained pointers to the disk blocks of the file. If the file contained more blocks than could fit in a directory entry, a second entry was used.

4.5.1 CD-ROM File Systems

File systems on cdroms do not need to support file addition or deletion and as a result have no need for free blocks. A CD-R (recordable) does permit files to be added, but they are always added at the end of the disk. The space allocated to a file is not recovered even when the file is deleted, so the (implicit) free list is simply the blocks after the last file recorded.

The result is that the file systems for these devices are quite simple.

The ISO9660 File System

This international standard forms the basis for essentially all file systems on data cdroms (music cdroms are different and are not discussed). Most Unix systems use iso9660 with the Rock Ridge extensions, and most windows systems use iso9660 with the Joliet extensions.

The ISO9660 standard permits a single physical CD to be partitioned and permits a cdrom file system to span many physical CDs. However, these features are rarely used and we will not discuss them.

Since files do not change, they are stored contiguously and each directory entry need only give the starting location and file length.

File names are 8+3 characters (directory names just 8) for iso9660-level-1 and 31 characters for -level-2. There is also a -level-3 in which a file is composed of extents which can be shared among files and even shared within a single file (i.e. a single physical extent can occur multiple times in a given file).

Directories can be nested only 8 deep.

Rock Ridge Extensions

The Rock Ridge extensions were designed by a committee from the unix community to permit a unix file system to be copied to a cdrom without information loss.

These extensions included.

  1. The unix rwx bits for permissions.
  2. Major and Minor numbers to support special files, i.e. including devices in the file system name structure.
  3. Symbolic links.
  4. An alternate (long) name for files and directories.
  5. A somewhat kludgy work around for the limited directory nesting levels.
  6. Unix timestamps (creation, last access, last modification).

Joliet Extensions

The Joliet extensions were designed by Microsoft to permit a windows file system to be copied to a cdrom without information loss.

These extensions included.

  1. Long file names.
  2. Unicode.
  3. Arbitrary depth of directory nesting.
  4. Directory names with extensions.

4.5.2 The MS-DOS (and Windows FAT) File System

We discussed this linked-list, File-Allocation-Table-based file system previously. Here we add a little history.

MS-DOS and Early Windows

The FAT file system has been supported since the first IBM PC (1981) and is still widely used. Indeed, considering the number of cameras and MP3 players, it is very widely used.

Unlike CP/M, MS-DOS always had support for subdirectories and metadata such as date and size.

File names were restricted in length to 8+3.

As described above, the directory entries point to the first block of each file and the FAT contains pointers to the remaining blocks.

The free list was supported by using a special code in the FAT for free blocks. You can think of this as a bitmap with a wide bit.

The first version FAT-12 used 12-bit block numbers so a partition could not exceed 212 blocks. A subsequent release went to FAT-16.

The Windows 98 File System

Two changes were made: Long file names were supported and the file allocation table was switched from FAT-16 to FAT-32. These changes first appeared in the second release of Windows 95.

Long File Names

The hard part of supporting long names was keeping compatibility with the old 8+3 naming rule. That is, new file systems created with windows 98 using long file names must be accessible if the file system is subsequently used with an older version of windows that supported only 8+3 file names. The ability for new old systems to read data from new systems was important since users often had both new and old systems and kept many files on floppy disks that were used on both systems. This abiliity called backwards compatibility.

The solution was to permit a file to have two names: a long one and an 8+3 one. The primary directory entry for a file in windows 98 is the same format as it was in MS-DOS and contains the 8+3 file name. If the long name fits the 8+3 format, the story ends here.

If the long name does not fit in 8+3, an 8+3 version is produce via an algorithm that works but produces names with severely limited aesthetic value. The long name is stored in one or more axillary directory entries adjacent to the main entry. These axillary entries are set up to appear invalid to the old OS, which therefore ignores them.


FAT-32 used 32 bit words for the block numbers (actually, it used 28 bits) so the FAT could be huge (228 entries). Windows 98 kept only a portion of the FAT-32 table in memory at a time.

4.5.3 The Unix V7 File System

I presented the inode system in some detail above. Here we just describe a few properties of the filesystem beyond the inode structure itself.

4.6 Research on File Systems


4.6 Summary


Chapter 5 Input/Output

5.1 Principles of I/O Hardware

5.1.1 I/O Devices

The most noticeable characteristic of current ensemble of I/O devices is their great diversity.

5.1.2 Device Controllers

These are the devices as far as the OS is concerned. That is, the OS code is written with the controller specification in hand not with the device specification.

5.1.3 Memory-Mapped I/O (vs I/O Space Instructions)

Consider a disk controller processing a read request. The goal is to copy data from the disk to some portion of the central memory. How is this to be accomplished?

The controller contains a microprocessor and memory, and is connected to the disk (by wires). When the controller requests a sector from the disk, the sector is transmitted to the control via the wires and is stored by the controller in its memory.

The separate processor and memory on the controller gives rise to two questions.

  1. How does the OS request that the controller, which is running on another processor, perform an I/O and how are the parameters of the request transmitted to the controller?
  2. How are the data from a disk read moved from the controller's memory to the general system memory? Similarly, how is the write data moved to the controller?

Typically the interface the OS sees consists of some several registers located on the controller.

So the first question above becomes, how does the OS read and write the device register?

5.1.4 Direct Memory Access (DMA)

We now address the second question, moving data between the controller and the main memory. Recall that (independent of the issue with respect to DMA) the disk controller, when processing a read request pulls the desired data from the disk to its own buffer (and pushes data from the buffer to the disk when processing a write).

Without DMA, i.e., with programmed I/O (PIO), the cpu then does loads and stores (assuming the controller buffer is memory mapped, or uses I/O instructions if it is not) to copy the data from the buffer to the desired memory locations.

A DMA controller, instead writes the main memory itself, without intervention of the CPU.

Clearly DMA saves CPU work. But this might not be important if the CPU is limited by the memory or by system buses.

An important point is that there is less data movement with DMA so the buses are used less and the entire operation takes less time. Compare the two blue arrows vs. the single red arrow.