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Directories

As with other modern file systems, a directory in NTFS is a file whose data contains a collection of name/file mappings.

  • A directory entry contains the name of the file and file reference. The file references identify the file on this volume. In other words,it is an internal name for the file.

A reference is a (file number, sequence number) pair. The file number is the offset of the file's entry in the MFT table. It issimilar to the Unix inumber (Inode number).

  • The list of file names in the directories is not stored in a simple list, but rather as a lexigraphically-sorted tree, called a B+ tree (thiswill be familiar to those with a database background). The data structure is called an index in NFTS (again, following the terminology from databases).
  • The NTFS design specifies that an index can be constructed for any attribute, but currently only file name indices are supported.
  • The name for a file appears both in its directory entry and in the MFT entry for the file itself.
  • As with regular files, if the directory is small enough, it can fit entirely within the MFT entry.

If the directory is larger, then the top part of (the B+ tree of) the directory is in the MFT entry, which points to extents thatcontain the rest of the name/file mappings.

File system crash recovery

Unix file system crash recovery

Computers can crash at any time, and we want the file system to behave sensibly in the face of crashes. The key idea is calledconsistency:

  • The file data and the various control structures (descriptors, bitmaps) must be in agreement.
  • Since crashes can occur at any time, not all updates to the disk may be completed.
  • We must insure that when the system reboots, it can return its file system to some sensible state.
  • The key constraint is that any file system write operation, in progress at the time of the crash, either completely finishes or appears as ifit never happened. This is called atomicity by the database folks.

Insuring consistency requires two things:

  • Updates to the file system data structures must be done in the write order (and there is only one right order)!
  • The proper steps must be taken at reboot time to bring the system back in to a consistent state.

There are three basic updates that happen when data is written to a file.

  1. A block (or blocks) is allocated from the free list (bit map).
  2. Data is written to the newly allocated block.
  3. The inode is updated to include the new data.

These operations must be done in the above order. If they are not, then it is possible to have a data block included in a filethat might have garbage (uninitialized data) in the block.

After rebooting, the recovery utility program on Unix, called "fsck", is going to traverse the entire directory structure of thedisk to insure that all free blocks are in the free list.

Recovery after a crash follows these steps:

  1. Allocate a temporary bit map, initialized to indicate that all disk blocks are free.
  2. Start at the inode for the root directory.
  3. Traverse the directory:
    • For each disk data block in the directory file, marks its blocks as "allocated" in the bit map.
    • For each data file in this directory, marks its data blocks as "allocated" in the bit map.
    • For each directory in this directory, perform the "Traverse the directory" stepsabove.

At the completion of the algorithm, you can compare the actual bit map to the temporary one to find blocks that were allocated, butnever made it into a file.

Windows file system crash recovery

NTFS assures that the file system will remain consistent by use of a write log. This technique is similar to that used in adatabase system.

As in other file systems, consistency means that a write (or group of writes) to a file either complete or do not happen at all. Itis not possible for a data block to be in an undefined state (e.g., allocated, but not written).

  • The log is one of those standard files stored at the beginning of the MFT. It is called, cleverly enough, the log file.
  • A simplified version of the steps to write data to a file look like:
    • A file update is written to the in-memory log buffer.
    • Updates to the in-memory file data and associated file system structures are made.
    • The log changes are flushed to disk.
    • The file data and structure changes are flushed to disk.
  • If the system crashes during a file update, it is sufficient to go through the log an re-do each operation specified in the log.
  • The system occasionally creates checkpoints, so that it does not have to back to the beginning of the log for recovery. Checkpoints have two mainbenefits:
  • Log files can be truncated, reducing the space needed for the log.
  • Recovery time is faster if fewer log records need to be processed.

Disk scheduling

Disk scheduling: in a system with many processes running, it can often be the case that there are several disk I/O's requested atthe same time. The order in which the requests are serviced may have a strong effect on the overall performance of the disk.

First come first served (FIFO, FCFS): may result in a lot of unnecessary disk arm motion under heavy loads.

Shortest seek time first (SSTF): handle nearest request first. This can reduce arm movement and result in greateroverall disk efficiency, but some requests may have to wait a long time.

Scan: like an elevator. Move arm back and forth, handling requests as they are passed. This algorithm does notget hung up in any one place for very long. It works well under heavy load, but not as well in the middle (about 1/2 the time it will not get the shortestseek).

Minor variant: C-SCAN , which goes all the way back to front of disk when it hits the end, sort of like araster scan in a display.

LOOK algorithm. Like scan, but reverse direction when hit the last request in the current direction. C-LOOK isthe circular variant of LOOK. What most systems use in practice.

Questions & Answers

anyone know any internet site where one can find nanotechnology papers?
Damian Reply
research.net
kanaga
Introduction about quantum dots in nanotechnology
Praveena Reply
what does nano mean?
Anassong Reply
nano basically means 10^(-9). nanometer is a unit to measure length.
Bharti
do you think it's worthwhile in the long term to study the effects and possibilities of nanotechnology on viral treatment?
Damian Reply
absolutely yes
Daniel
how to know photocatalytic properties of tio2 nanoparticles...what to do now
Akash Reply
it is a goid question and i want to know the answer as well
Maciej
characteristics of micro business
Abigail
for teaching engĺish at school how nano technology help us
Anassong
Do somebody tell me a best nano engineering book for beginners?
s. Reply
there is no specific books for beginners but there is book called principle of nanotechnology
NANO
what is fullerene does it is used to make bukky balls
Devang Reply
are you nano engineer ?
s.
fullerene is a bucky ball aka Carbon 60 molecule. It was name by the architect Fuller. He design the geodesic dome. it resembles a soccer ball.
Tarell
what is the actual application of fullerenes nowadays?
Damian
That is a great question Damian. best way to answer that question is to Google it. there are hundreds of applications for buck minister fullerenes, from medical to aerospace. you can also find plenty of research papers that will give you great detail on the potential applications of fullerenes.
Tarell
what is the Synthesis, properties,and applications of carbon nano chemistry
Abhijith Reply
Mostly, they use nano carbon for electronics and for materials to be strengthened.
Virgil
is Bucky paper clear?
CYNTHIA
carbon nanotubes has various application in fuel cells membrane, current research on cancer drug,and in electronics MEMS and NEMS etc
NANO
so some one know about replacing silicon atom with phosphorous in semiconductors device?
s. Reply
Yeah, it is a pain to say the least. You basically have to heat the substarte up to around 1000 degrees celcius then pass phosphene gas over top of it, which is explosive and toxic by the way, under very low pressure.
Harper
Do you know which machine is used to that process?
s.
how to fabricate graphene ink ?
SUYASH Reply
for screen printed electrodes ?
SUYASH
What is lattice structure?
s. Reply
of graphene you mean?
Ebrahim
or in general
Ebrahim
in general
s.
Graphene has a hexagonal structure
tahir
On having this app for quite a bit time, Haven't realised there's a chat room in it.
Cied
what is biological synthesis of nanoparticles
Sanket Reply
what's the easiest and fastest way to the synthesize AgNP?
Damian Reply
China
Cied
types of nano material
abeetha Reply
I start with an easy one. carbon nanotubes woven into a long filament like a string
Porter
many many of nanotubes
Porter
what is the k.e before it land
Yasmin
what is the function of carbon nanotubes?
Cesar
I'm interested in nanotube
Uday
what is nanomaterials​ and their applications of sensors.
Ramkumar Reply
how did you get the value of 2000N.What calculations are needed to arrive at it
Smarajit Reply
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Source:  OpenStax, Operating systems. OpenStax CNX. Aug 13, 2009 Download for free at http://cnx.org/content/col10785/1.2
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