Operating Systems

Start Lecture #12

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).

• If the system crashed, it is possible that not all metadata was written to disk. As a result the file system may be inconsistent. These programs check, and often correct, inconsistencies.
• Scan all i-nodes (or fat) to check that each block is in exactly one file, or on the free list, but not both.
• Also check that the number of links to each file (part of the metadata in the file's i-node) is correct (by looking at all directories).
• Other checks as well.
• Offers to fix the errors found (for most errors).

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

Caching

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?

• The system must update the buffer cache if the file block is present (otherwise subsequent reads will return the old value).
• If the file block is not present, then a cache block is allocated for it, which likely causes an old file block to be evicted.
  This decision to allocate a cache block for writes that are not present in the cache is called a write-allocate policy Although no-write-allocate is possible and sometimes used for memory caches, it performs poorly for disk caching.
• The major question is whether the system should write the new value of the block to disk in addition to updating the buffer cache.
• The simplest alternative is write through in which each write is performed at the disk as well.
  • Since floppy disk drivers adopt a write-through policy, one can remove a floppy as soon as an operation is complete.
  • Write through results in heavy I/O write traffic.
    • If a block is written many times all the writes are sent the disk. Only the last one was needed.
    • If a temporary file is created, written, read, and deleted, all the disk writes were wasted.
  • No modern system uses write-through for hard drives.
• The other alternative is write back in which the disk is not updated until the in-memory copy is evicted (i.e., at replacement time).
  • Much less write traffic than write through. Hence all modern systems use write back for hard drives.
  • Trouble if a crash occurs or if the disk is removed before the write back completes (think of a removable flash drive).
  • The system is permitted to write dirty blocks back before they are evicted. It is common for a system to write back all dirty blocks about once a minute. This limits the possible damage, but also the possible gain.
  • Ordered writes. Do not write a block containing pointers until the blocks pointed to have been written. This is especially important if the block pointed to contains pointers since the version of these pointers on disk may be wrong and you are giving a file pointers to some random blocks.

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.

• How does the system decide that the access pattern looks sequential?
  • If a seek system call is issued, the access pattern is not sequential.
  • If a process issues consecutive read() system calls for block n-1 and then n, the access patters is guessed to be sequential.
• What if block n+1 is already in the block cache?
  Ans: Don't issue the read ahead.
• Would it be reasonable to read ahead two or three blocks?
  Ans: Yes.
• Would it be reasonable to read ahead the entire file?
  Ans: No, it could easily pollute the cache evicting needed blocks, and could waste considerable disk bandwidth.

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.

4.5 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

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.

• Each directory entry contains a name and a pointer to the corresponding i-node.
• The metadata for a file or directory is stored in the corresponding inode.
• Early unix limited file names to 14 characters, stored in a fixed length field.
• The name field now is of varying length and file names can be quite long. On my linux system

          touch 255-char-name
        

  is OK but

          touch 256-char-name
        

  is not.
• To go down a level in the directory hierarchy takes two steps: get the inode, get the file (or subdirectory).
• This shows how important it is not to parse filenames for each I/O operation, i.e., why the open() system call is important.
• Do on the blackboard the steps for /a/b/X

4.6 Summary

Read

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.

• Some, e.g. disks, transmit megabytes per second; others, like keyboards, don't transmit a megabyte in their lifetime.
• Some, like ethernet, are purely electronic; others, like disks are mechanical marvels.
• A mouse can be put in your pocket; a high-speed printer needs at least two people to move it.
• Some devices, e.g. a keyboard, are input only. But note that this is in some sense a crazy classification since a keyboard produces output, which is sent to the computer, and receives very little input from the computer (mostly to turn on/off a few lights). Really a keyboard is a transducer, taking mechanical input from a human and producing electronic output for a computer.
• Similarly, an output only device such as a printer supplies very little output to the computer (perhaps an out of paper indication) but receives voluminous input from the computer. Again it is better thought of as a transducer, converting electronic data from the computer to paper data for humans.
• Many devices are input/output, but again the words can be funny. A disk is viewed as an input device when it is being read, i.e., when it is outputting data; it is viewed as an output device when it is inputting data.
• Often devices are characterized as block devices or as character devices. The distinction being that devices like disks are read/written in blocks and individual blocks can be addressed (i.e., not all accesses are sequential). An ethernet interface or a printer has no notion of block or addresses. Instead, it just deals with a stream of characters.
• However, the block/character device distinction is fuzzy. What about tapes? They read/write blocks and are sort of block addressable (rewind then skip forward). Clocks are weird and hard to classify; they simply generate periodic interrupts.

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.

• Controllers are also called adaptors.
• A controller abstracts away some of the low level features of the devices it controls. For example, a disk controller performs error checking and assembles the bit stream coming off the disk into blocks of bytes.
• In the old days controllers handled interleaving of sectors. Sectors are interleaved if the controller or CPU cannot handle the data rate and would otherwise have to wait a full revolution between sectors. This is not a concern with modern systems since the electronics have increased in speed faster than the devices and now all disk controllers can handle the full data rate of the disks they support.
• Graphics controller do a great deal. They often contain processors at least as powerful as the main CPU on which the OS runs.

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.

• These are memory locations into which the OS writes information such as the sector to access, read vs. write, length, where in system memory to put the data (for a read) or from where to take the data (for a write).
• There is also typically a device register that acts as a go button.
• There are also devices registers that the OS reads, such as the status of the controller, any errors that were detected, etc.

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

• With memory-mapped I/O the device registers appear as normal memory. All that is needed is to know at which address each device register appears. Then the OS uses normal load and store instructions to read and write the registers.
• Some systems instead have a special I/O space into which the registers are mapped. In this case special I/O space instructions are used to accomplish the loads and stores.
• From a conceptual point of view there is no difference between the two models, but the implementations differ. For example.
  1. Memory-mapped I/O is a more elegant solution in that it uses an existing mechanism to accomplish a second objective.
  2. Since normal loads and stores are used for memory-mapped I/O, the algorithm can be written in a high-level language. Assembly language is needed for I/O space instructions since such instructions cannot be expressed in (normal) high-level languages.
  3. A memory-mapped implementation must arrange that these addresses are sent to the appropriate bus and must insure that they are not cached.

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.

Since PIO is pure software it is easier to change, which is an advantage.

DMA does need a number of bus transfers from the CPU to the controller to specify the DMA. So DMA is most effective for large transfers where the setup is amortized.

Why have the buffer? Why not just go from the disk straight to the memory?

1. Speed matching.
   The disk supplies data at a fixed rate, which might exceed the rate the memory can accept it. In particular the memory might be busy servicing a request from the processor or from another DMA controller.
   Alternatively, the disk might supply data at a slower rate than the memory (and memory bus) can handle thus under-utilizing an important system resource.
2. Error detection and correction.
   The disk controller verifies the checksum written on the disk.

Homework: 12