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1
File Systems
Chapter 6
6.1 Files 6.2 Directories 6.3 File system implementation 6.4 Example file systems
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Long-term Information Storage
1. Must store large amounts of data
2. Information stored must survive the termination of the process using it
3. Multiple processes must be able to access the information concurrently
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Logische Struktur von Dateien• Dateiarten
– Folge von Bytes• generell “Bytefolge”• Bytes codieren speziellen Inhalt in einer Weise, die
dem Dateisystem nicht bekannt ist– Folge von Datensätzen (Records)
– Menge von Datensätzen mit Schlüsselattribut– (Baum aus verzeigerten
Datensätzen)
Name Straße PLZ Ort Id-Nr Name Straße PLZ Ort Id-Nr
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File Naming
Typical file extensions.
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File Types
(a) An executable file (b) An archive
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Dateizugriffsorganisation• Sequentiell
– strikt von vorn nach hinten lesen / schreiben
• Wahlfrei– Bytes / Datensätze sind adressierbar
• per Nummer / Adresse• per Schlüsselattribut-Wert
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Datei: Verwalteter Verbund aus DatenDatei: Menge von Attributen
– beschreibend– einordnend– Speicherung verwaltend– die Daten
– prominente Attribute• Name• Länge• Inhalt• Erzeugungsdatum• Letztes Änderungsdatum• Dateischutz• Verweis in Verwaltungsstruktur
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File Attributes
Possible file attributes
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Datei: Extern gespeicherte Datensammlung
Der effiziente Zugriff muss vorbereitet werden:• Verwaltungsinformation im Arbeitsspeicher
ablegen• Puffer zum Zugriff einrichten• Geräte belegen
öffnen
schließen
positionieren
lesen
schreiben
Attribute schreiben
Erzeugen
Löschen
Attribute lesen
Länge ändern
InhaltszugriffeDatei alsGanzes
und
Nicht-Inhalts-attributezugreifen
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An Example Program Using File System Calls (1/2)
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An Example Program Using File System Calls (2/2)
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DirectoriesSingle-Level Directory Systems
• A single level directory system– contains 4 files– owned by 3 different people, A, B, and C
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Two-level Directory Systems
Letters indicate owners of the directories and files
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Hierarchical Directory Systems
A hierarchical directory system
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15A UNIX directory tree
Path Names
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Directory Operations
1. Create2. Delete3. Opendir4. Closedir
5. Readdir6. Rename7. Link8. Unlink
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File System Implementation
A possible file system layout
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Datei-SpeicherungWie werden die Elemente einer Datei auf dem
Speichermedium positioniert (und bei Zugriffen gefunden)?
• Zusammenhängende Speicherung– Adressrechnung zur Positionierung möglich– Einfache Verwaltung– Lücken, fragliche Verlängerbarkeit
• Gestückelte Speicherung– Verzeigerte Liste– Hierarchische Verzeigerung (z.B. Indexsequentiell)– In Tabelle verwaltete Stücke– Verzeigerte Tabellenelemente (hierarchisch, z.B. i-node)
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Implementing Files (1)
(a) Contiguous allocation of disk space for 7 files(b) State of the disk after files D and E have been removed
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Implementing Files (2)
Storing a file as a linked list of disk blocks
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Implementing Files (3)
Linked list allocation using a file allocation table in RAM
+ Verfolgen der Verweisliste zum Positionieren ohne Plattenzugriffe möglich, da File Allocation Table (FAT) im Arbeitspeicher
- 1 FAT für alle Dateien einer Platte ist sehr groß und nimmt viel Arbeitsspeicher weg.
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Implementing Files (4)
An example i-node
i-node enthält die Verweise einer einzigen Datei und befindet sich auf der Platte:
• Nach Einlesen in Arbeitsspeicher kann effizient positioniert werden
• Nur i-nodes der offenen Dateien sind im Arbeitsspeicher
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Implementing Directories (1)
(a) A simple directoryfixed size entriesdisk addresses and attributes in directory entry
(b) Directory in which each entry just refers to an i-node
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Implementing Directories (2)
• Two ways of handling long file names in directory– (a) In-line– (b) In a heap
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Shared Files (1)
File system containing a shared file
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Shared Files (2)
(a) Situation prior to linking(b) After the link is created(c) After the original owner removes the file
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Disk Space Management (1)
• Dark line (left hand scale) gives data rate of a disk• Dotted line (right hand scale) gives disk space efficiency• All files 2KB
Block size
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Disk Space Management (2)
(a) Storing the free list on a linked list(b) A bit map
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Disk Space Management (4)
Quotas for keeping track of each user’s disk use
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File System Reliability (1)
• A file system to be dumped– squares are directories, circles are files– shaded items, modified since last dump– each directory & file labeled by i-node number
File that hasnot changed
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File System Reliability (2)
Bit maps used by the logical dumping algorithm
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File System Reliability (3)
• File system management states(a) consistent(b) missing block(c) duplicate block in free list(d) duplicate data block
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File System Performance (1)
The block cache data structures
Strategie-Zeiger
(z.B. LRU)
Hash-Kollisionsauflösungsliste
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File System Performance (2)
• I-nodes placed at the start of the disk(langer Weg vom i-node zum ersten Inhaltsblock)
• Disk divided into cylinder groups– each with its own blocks and i-nodes
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Log-Structured File Systems• With CPUs faster, memory larger
– disk caches can also be larger– increasing number of read requests only accesses cache– thus, most real disk accesses will be writes
• LFS Strategy structures entire disk as a log– have all writes initially buffered in memory– periodically write these to the end of the disk log– when file opened, locate i-node, then find blocks
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Example File Systems CD-ROM File Systems
The ISO 9660 directory entry
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The MS-DOS File System (1)
The MS-DOS directory entry
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The MS-DOS File System (2)
• Maximum partition for different block sizes• The empty boxes represent forbidden combinations
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The Windows 98 File System (1)
The extended MOS-DOS directory entry used in Windows 98
Bytes
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The Windows 98 File System (2)
An entry for (part of) a long file name in Windows 98
Bytes
Checksum
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The Windows 98 File System (3)
An example of how a long name is stored in Windows 98
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The UNIX V7 File System (1)
A UNIX V7 directory entry
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The UNIX V7 File System (2)
A UNIX i-node
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The UNIX V7 File System (3)
The steps in looking up /usr/ast/mbox