Nachos Instructional OS: Part 3B CS 170, Tao Yang Topics File - - PowerPoint PPT Presentation

Nachos Instructional OS: Part 3B

CS 170, Tao Yang

5/26/2015 2

Topics

 File system implementation.  Assignment 3B for File system

extension

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Nachos File System



Two versions:

 A ``stub'' version is simply a front-end to the Linux.  A Nachos version implemented on top of a raw disk.



Function layers (from top to down):

 FileSystem object

 Directory support.  File control block (I-node) support  Free block map

 OpenFile objects (many)

 Created by every call to open/access a file in a directory.

 SynchDisk object

 Created by synchDisk = new SynchDisk("DISK") in system.cc

 The Disk object.

 Created by disk = new Disk(name,..) in SynchDisk::SynchDisk()

Nachos Kernel Read/write

• penFile objects

SynchDisk Disk Linux

• penFile objects
• penFile objects

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Disk Size



Machine/disk.h #define SectorSize 128 // number of bytes per disk sector #define SectorsPerTrack 32 // number of sectors per disk track #define NumTracks 32 // number of tracks per disk #define NumSectors (SectorsPerTrack * NumTracks) // total # of sectors per disk

Nachos Kernel Read/write

• penFile objects

SynchDisk Disk

File System Layers in Nachos

Base Operating System (Linux for our class) FileSystem:

Directory management file create/open/delete

Disk: low-level interface that initiates

asynchronous IO to physical disk sectors

OpenFile: access individual file with read/write/seek.

Operate on bytes.

SynchDisk: synchronous access to a disk with concurrent threads.

Operate on sectors.

Nachos kernel thead

Directory: Manage a directory of file names FreeMap Bitmap for 32x32 disk sectors

FileHeader: i-node (File control block)

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Operations of File System Object



FileSystem(bool format).

 called once at ``boot'' time.  For the stub version, just use Unix file system  Otherwise create a directory/control info.

 Sector 0 is fileheader (i-node) for FreeMap.  Sector 1 is i-node for directory table.



Create(char *name, int initialSize).

 Create a Nachos disk file. ``initialSize'' is ignored in the

stub version; Unix doesn't require it.



OpenFile *Open(char *name).

 Assume both read and write access.  in the stub version, Open simply opens the Unix file.



Remove(char *name)



List()

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Directory Object contains 10 DirectoryEntry objects (directory.h)



Directory content has 10 table entries.  fit into a 128B sector.



Stored in Sector 1



Directory space is allocated in FileSystem() during initialization

… Bool inUse --- is this entry used? Int sector --- location of file header (i-node) Filename -- 9 characters+ 1 for ‘\n’ … 128 bytes

memory

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directory.cc



Limit



FileNameMaxLen 9 // 9 characters at most for a file name



NumDirEntries 10 // at most 10 files



Directory Object attributes



tableSize -- # of directory entries which is 10 now.



table = in-memory directory entries, each of which is for one Nachos file. Content is copied from and copies back to a disk DirectoryFile.



Operations



Directory (int size) -- Create an in-memory directory of the desired size.



FetchFrom (OpenFile *file) -- Fetch disk DirectoryFile that contains directory entries, and copy to in-memory directory table.



WriteBack(OpenFile *file) -- Write back in-memory directory content to disk DirectoryFile



called every time when a file is added or removed to directory.



FindIndex (Char *name) --Search a file string name in a directory



Add (Char *name, int sector) – Add file name into a directory with disk sector no that contains the file header (i-node).



Remove(char *name) – remove a file.

Directory Disk Directory Block

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filehdr.h: File header (File control block, i-node)

… int numBytes; // file size Data sector pointer 0 … int numSectors; //sectors used Data sector pointer 29 Data sector pointer 1 Data sector Data sector Data sector int dataSectors[NumDirect]



Fit in one sector (128 bytes)



Operations



Allocate(BitMap *bitMap, int fileSize);



FetchFrom (int SectorNo) -- Fetch Fileheader sector from disk.



WriteBack(int SectorNo) -- Write back in-memory file header content to disk



ByteToSector(int offset) –Convert a file offset in bytes to a sector no

memory File control block Disk File header

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OpenFile Object



OpenFile(int sector)

 sector – disk location of file control block.  Load FCB from disk

create an in-memory OpenFile object



Seek(int position)



Read/write in the current position



Read(char *buffer, int numBytes)



Write(char *buffer, int numBytes)



Read/write in a specific position



ReadAt(char *buffer, int numBytes, int FilePosition).



Identify specific sectors in the file and fetch them, copy to buffer.



WriteAt(char *buffer, int numBytes, int FilePosition)



May read partially written sectors first, and then write out updated sectors. 

Length(). Return the file size.

Nachos Kernel Read/write

• penFile objects

SynchDisk Memory File control block Disk OpenFile Obj

• penFile objects
• penFile objects

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SynchDisk Object: Synchronously access a disk



Filesys/synchDisk.cc



SynchDisk ()



Called only once to create a physical disk object. Set the disk interrput hander as RequstDone().



Create lock/semaphore for synchronization.



Notice that physical disk is an asynchronous device (disk requests return immediately, and an interrupt happens later on).



ReadSector(int sectorNumber, char* buffer)



Send a read request for a sector



Wait IO complete (wait for interrupt using a semaphone)



WriteSector(int sectorNumber, char* data)



Send a sector write request. Wait for interrupt.



RequestDone()



Disk interrupt handler. Wake up any thread waiting for the disk request to finish.

Nachos Kernel Read/write

• penFile objects

SynchDisk

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Disk Object



Machine/disk.h and disk.cc



Simulates the behavior of a disk I/O device.

 a single platter, with multiple tracks (32).  Each track contains 32 sectors  Each sector is 128 bytes  Disk size = 32x32*128 =128K  Asynchronous read/write with an option of disk cache



Supported operations:

 Disk(char *name, VoidFunctionPtr callWhenDone, int

callArg)

 Create a disk: just create a Unix file to simulate.  Setup a handling function (RequestDone in

SynchDisk.cc) when a read/write request is done.

Nachos Kernel Read/write

• penFile objects

SynchDisk Disk 128K

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Operations in Disk device



ReadRequest(int sectorNumber, char *buffer)

 Read from the specific location of the “physical disk”.  ticks = ComputeLatency(sectorNumber, FALSE);  interrupt->Schedule(DiskDone, (int) this, ticks, DiskInt);



WriteRequest(int sectorNumber, char *data)

 Write data to specific location of the “physical disk”.  ticks = ComputeLatency(sectorNumber, TRUE);  interrupt->Schedule(DiskDone, (int) this, ticks, DiskInt);



ComputeLatency(int newSector, bool writing)

 Latency= seek time + rotation time

Nachos Kernel Read/write

• penFile objects

SynchDisk Disk

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Assignment 3B Workload

 Test C programs ~cs170/nachos-

projtest/proj3

 Part B. ~200 in filesys (few in userprog)  Part A and B can be done in parallel

 Part B may use Project 2 code without VM.

 Work on filesys subdirectoy mainly.

 And change system call handling code in userprog

if needed (to use this extended file system).

 Extend the size of each file from 4K to 100K

 single indirect pointers in i-node (file control

block).

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Emphasis of Project 3B

 Play tradeoffs

 Understand the design options  Deadline-driven approach

 Choose something simpler and easier to do first.

 Still not enough time?

 Complete with minimal efforts to pass the tests.

 Hidden tests are very similar to public test cases. No

complex test cases.

 You are good as long as your code can run virtual

memory>physical, can support files up to 100K.

 Interpret the requirement towards your advantage

if vague. If not specified, do something easy.

Part B: Steps for Nachos file system extension

1.

Understand how the Nachos file system operates and how Nachos implements files.

2.

Modify the file system calls from Project 2 to use Nachos file system

1.

Donot have to support VM (Part A/Part B can be done in parallel)

3.

Modify Nachos file system so that the file size can be upto 100K

Files to be modified



Directory filesys



filehdr.cc/.h



Support 30 direct data pointers



New: Support single indirect pointers in i-node and can expand the file length with more sectors allocated.



Write method ExtendFile() that allows a file to grow by certain number of sectors.



• penfile.cc



Expand file length/allocate sectors when writing data at the end or beyond.



IndirectPointerBlock.cc (new file to add)

This class provides a way for a file header to contain not just direct sectors, but indirect blocks that each have their individual collection of

• sectors. This allows the file size to exceed the usual 4K.



Directory userprog



exception.cc



Make sure file system calls use extended Nachos file system.

Useful methods in IndirectPointerBlock.cc

// Constructor

IndirectBlock::IndirectBlock() //Writes the modified object back to the specified sector on disk. void IndirectBlock::WriteBack(int sectorNum) // Retrieves the serialized object from disk. void IndirectBlock::FetchFrom(int sectorNum) // Adds a newly allocated data sector from the file header to this indirect pointer block's list of allocated sectors. void IndirectBlock::PutSector(int sectorNum) // Removes the allocated sectors from the global map of allocated sectors. Essentially removes the data held by this indirect pointer block. void IndirectBlock::Deallocate(BitMap* allocSectorsMap) // Returns the disk sector that is corresponding the specified byte at

• ffset from this indirect pointer block

int IndirectBlock::ByteToSector(int offsetFromStartOfIB)

Comparison: Unix file i-node (4K bytes per block)

Step 3: Extend maximum file size

 Modify the FileHeader i-node (file control block) to

support 100K size.

 Option 1: Include 5 data sector pointers, and 25 sectors for

single indirect mapping.

 At most map 5 + 25*32=805 blocks. Maximum file

length = 805*128=103040bytes.

 Option 2: single indirect mapping at every entry

 Suggest to add a class called IndirectPointerBlock

which takes 1 sector (32 entries)

 FetchFrom (int sector). WriteBack(int sector).  AddSector (int sector) – add a new sector.  Deallocate(BitMap *freeMap) – deallocate all sectors.  ByteToSector (int offset). – convert an offset in bytes to a

sector.

Step 4: File size can grow

 Modify the current implementation so that a write

beyond the end of the file extends the size of the file.

 Gracefully handle the case of the disk being full or

maximum size is reached - the user process should not crash. Instead an error should be returned.

Step 4

 Modify WriteAt() method in openfile.cc :

 Case 1: Write request is within the bound of the

• file. Handle like the existing code

 Case 2: Write request overwrites some portion of

existing file and goes beyond by N bytes.

 Case 3: No overwrite to the current data, but goes

beyond the end of the file. Here.

 To extend a file dynamically,

 Extend i-node/data dynamically: include a routine

in FileHeader to extend a file by N bytes (by a sector or by a set of sectors)

Testing issues

 Need to load user binaries/data into Nachos file

system during execution



Example: $ pwd cs170/nachos/code/filesys $ ./nachos -f -cp ../test/Prog1 Prog1 -x Prog1

 Or

nachos –f nachos -cp Prog1 Prog1 nachos -x Prog1

Linux file system Nachos file system Nachos –cp Prog1 Prog1 Nachos –x Prog1 Nachos –f