COMP310代做、代写Programming、代做C++语言、C/C++编程设计调试

日期：2019-11-24 07:47

ECSE427/COMP310 (Revision 6) Page 1 of 7

Programming Assignment #3: Mountable Simple File System

Due date: Check My Courses`

1. What is required as part of this assignment?

In this assignment, you are expected to design and implement a simple file system (SFS) that can be

mounted by the user under a directory in the user’s machine. You need to demonstrate a file system

working in Linux. It may work with some modifications in other Unix-like operating systems (e.g., OSX). To simplify the assignment, SFS introduces many limitations such as restricted filename lengths, no

user concept, no protection among files, no support for concurrent access, etc. This is a design assignment.

Therefore, several aspects of the SFS are left undefined. It is your responsibility to make valid assumptions

and document them. No matter what assumptions you make, your implementation should work with the

testers and also it should be mountable through FUSE to get full marks.

Here is a list of restrictions we propose for SFS. You could add more provided you keep the SFS

functionally compatible with the testers and FUSE. You need to document all the simplifying assumptions

you make.

• Limited length filenames (select an upper limit such as 16)

• Limited length file extensions (could be set to 3 – following the common extension length)

• No subdirectories (only a single root directory – this is a severe restriction – relaxing this would

enable your file system to run many applications)

• Your file system is implemented over an emulated disk system, which is provided to you.

Here is a schematic that illustrates the overall concept of SFS in direct mode. You need to implement the

yellow (orange) component. The other two components (tester and disk) are given to you.

Figure 1: SFS in direct mode.

The problem with the direct mode is that SFS is only usable by your program. You may not be happy about

it! Why not use SFS to store data created by an editor like vi? The mountable version of SFS addresses

this problem. It allows you to store anything and treat SFS like an extension of the OS. The magic is

performed by FUSE – file systems in user space. The concept of FUSE is similar to the concept we saw in

micro kernel OSes. I said it is similar to micro-kernel OS (it is actually not).

Simple File

Figure 2: SFS in mounted mode.

The gray colored modules in the above schematic are provided by the Linux OS. The blue colored modules

are given to you as part of the support code provided as part of the assignment.

2. Objectives in detail

The file system you implement will be tested in two stages: direct mode and mounted mode. In the

direct mode, it is compiled and linked with a test suite that just calls the API functions to create files, write,

and read data. It performs various integrity checks for the file operations starting from simple to complex.

The test suite will count errors and report you the number of errors detected while exercising the API

functions. Ideally, you should debug your implementation to reach zero errors as reported by the test suite.

Even in that state your implementation can have many more errors that are not detected by our test suite.

We will not evaluate your implementation using a private test suite that is not released to you. So, the errors

that are not detected by the test suite will remain dormant and some of them could make your file system

unstable in the mounted mode. You could lose few points (less than 5) if those dormant bugs get activated

by FUSE calls!

We strongly suggest that you support the following API or something that is closely equivalent to this

in your implementation. You may deviate slightly, however, it is necessary to keep the functionality the

same. One of the unique aspects of the file system as specified below is the independent read and write

pointers. When a file is opened (we have only one mode which is read and write), the read file pointer is at

the beginning of the file and write pointer is at the end of the file. That is, the file is setup for appending

data to it. If the file is new, both pointers are at the beginning of the file. As you write data to the file, only

the write pointer moves. As you read, the read pointer moves. So, we can read a file sequentially, without

manipulating (or repositioning) the read pointer. Similarly, we can just append to a file without

manipulating the write pointer. By manipulating the write pointer, we could write data in other places like

over writing some existing data or even writing to a remote new location like shown in the code fragment

below. Note that the code below does not use the API of your filesystem so it is shown for illustration

purposes. If you run this in different “Unix” operating systems, you will see different behaviors!

#include <fcntl.h>

#include <unistd.h>

#include <sys/types.h>

#include <sys/uio.h>

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int fd = open("hello.dat", O_CREAT|O_RDWR);

lseek(fd, 121323434, SEEK_SET);

write(fd, buf, 1);

close(fd);

}

You can compile and run the program and then do “ls –ls”. This should show two numbers: length of

the file and the number of blocks used (first number displayed). In some systems (Linux), that number

would be very small (around 130). This indicates a sparse file. In OS X, it seems like a dense file where the

file system is using blocks to represent many uninitialized blocks. You need to follow the sparse approach.

Don’t use blocks for the blank data that goes into all intermediate blocks.

The C based API for the SFS is given below. It is strongly suggested that you retain the functionality

provided by the API if you decide to change it. That is, your API needs to provide equivalent functions.

void mksfs(int fresh); // creates the file system

int sfs_getnextfilename(char *fname); // get the name of the next file in directory

int sfs_getfilesize(const char* path); // get the size of the given file

int sfs_fopen(char *name); // opens the given file

int sfs_fclose(int fileID); // closes the given file

int sfs_frseek(int fileID,

int loc); // seek (Read) to the location from beginning

int sfs_fwseek(int fileID,

int loc); // seek (Write) to the location from beginning

int sfs_fwrite(int fileID,

char *buf, int length); // write buf characters into disk

int sfs_fread(int fileID,

char *buf, int length); // read characters from disk into buf

int sfs_remove(char *file); // removes a file from the filesystem

The mksfs() formats the virtual disk implemented by the disk emulator and creates an instance of the

simple file system on top of it. The mksfs() has a fresh flag to signal that the file system should be created

from scratch. If flag is false, the file system is opened from the disk (i.e., we assume that a valid file system

is already there in the file system. The support for persistence is important so you can reuse existing data

or create a new file system.

The sfs_getnextfilename(char *fname) copies the name of the next file in the directory into fname

and returns non zero if there is a new file. So, this function is a directory walker. Once all the files have

been returned, this function returns 0. So, you should be able to use this function to loop through the

directory. In implementing this function, you need to ensure that the function remembers the current

position in the directory at each call. Remember in SFS we have a single-level directory. The

sfs_getfilesize(char *path) returns the size of a given file.

The sfs_fopen() opens a file and returns an integer that corresponds to the index of the entry for the

newly opened file in the open file descriptor table. If the file does not exist, it creates a new file and sets its

size to 0. If the file exists, the file is opened in append mode (i.e., set the write file pointer to the end of the

file and read at the beginning of the file). The sfs_fclose() closes a file, i.e., removes the entry from the

open file descriptor table. On success, sfs_fclose() should return 0 and a negative value otherwise. The

sfs_fwrite() writes the given number of bytes of buffered data in buf into the open file, starting from the

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current write file pointer. This in effect could increase the size of the file by the given number of bytes (it

may not increase the file size by the number of bytes written if the write pointer is located at a location

other than the end of the file). The sfs_fwrite() should return the number of bytes written. The

sfs_fread() follows a similar behavior. The sfs_rfseek() moves the read pointer and sfs_wfseek() moves

the write pointer to the given location. It returns 0 on success and a negative integer value otherwise. The

sfs_remove() removes the file from the directory entry, releases the file allocation table entries and releases

the data blocks used by the file, so that they can be used by new files in the future.

A file system is somewhat different from other components because it maintains data structures in

memory as well as disk! The disk data structures are important to manage the space in disk and allocate and

de-allocate the disk space in an intelligent manner. Also, the disk data structures indicate where a file is

allocated. This information is necessary to access the file.

3. Implementation strategy

The disk emulator given to you provides a constant-cost disk (CCdisk). This CCdisk can be considered

as an array of sectors (blocks of fixed size). You can randomly access any given sector for reading or

writing. The CCdisk is implemented as a file on top of an underlying file system. Therefore, the data you

store in the CCdisk is persistent across program invocations. To mimic the real disk, the CCdisk is divided

into sectors of fixed size. For example, we can split the space into 1024-byte sectors. The number of sectors

times the size of a sector gives the total size of the disk. In addition to holding the actual file and directory

data, we need to store auxiliary data (meta data) that describes the files and directories in the disk. The

structure and number of bytes spent on meta data storage depends on the file system design, which you

create as part of this assignment.

On-disk data structures of the file system include a “super” block, the root directory, free block list,

and i-Node table. The figure below shows a schematic of the on-disk organization of SFS. This is a

suggested organization of the on-disk data structures. You can make improvements to it.

The super block defines the file system geometry. It is also the first block in SFS. So, the super block

needs to have some form of identification to inform the program what type of file system format is followed

for storing the data. The figure below shows the proposed structure for the super block. We expect your file

system to implement these features, but some modifications are acceptable provided they are well

documented. Each field in the figure is 4 bytes long. For instance, the magic number field is 4 bytes long.

With a 1024-byte long block (recommended size), we can see that there will be plenty of unused space in

the super block.

Super&

Block I-node&Table Data&Blocks Free&BitMap

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A file or directory in SFS is defined by an i-Node. Remember we simplified the SFS by just having a

single root directory (no subdirectories). This root directory is pointed to by an i-Node, which is pointed to

by the super block (that is the super block remembers the i-Node of the root directory). The i-Node structure

we use here is slightly simplified too. It does not have the double and triple indirect pointers. It has direct

and single indirect pointers. All meta data information (size, mode, ownership) can be associated with the

i-Node. So, the directory entry can be pretty simple (i.e., no need to put meta data there). The figure below

shows the simplified i-Node structure.

We are suggesting the i-Node structure shown above to maintain a semblance of similarity to the UNIX

file system. However, the simplification made to the SFS i-Nodes already makes it impossible to read or

write the SFS using UNIX software or vice-versa.

The directory is a mapping table to convert the file name to the i-Node. Remember a file name can

have an extension too. You can limit the extension to 3 characters max. The file name (without extension)

could be limited as well (16 characters is suggested). A directory entry is a structure that contains two fields

(at least): i-Node and file name. You could add other fields (if you find necessary). Depending on the

number of entries you have in the directory, the directory could be spanning across multiple blocks in the

disk. The i-Node pointing to the root directory is stored in the super block so we know how to access the

root directory. We assume that the SFS root directory would not grow larger than the max file size we could

accommodate in SFS. Because we don’t double or triple indirect disk pointers, the max file size is somewhat

limited, but it is still not an issue for this assignment!

In addition to the on-disk data structures, we need a set of in-memory data structures to implement the

file system. The in-memory data structures improve the performance of the file system by caching the ondisk

information in memory. Two data structures should be used in this assignment: directory table and iNode

cache. The directory table keeps a copy of the directory block in memory. Don’t make the

simplification of limiting the root directory to a single block (this would severely restrict the size of the disk

– by limiting the number of files in disk). Instead, you could either read the whole directory into the memory

or have a cache for the currently used directory block. The later one could be hard to get right.

Further, when you want to create, delete, read, or write a file, first operation is to find the appropriate

directory entry. Therefore, directory table is a highly accessed data structure and is a good candidate to keep

in memory. Another data structure to cache in the memory is the free block list. See the class notes for

different implementation strategies for the free block list.

Magic(0xACBD0005)

BlockSize(1024)

FileSystemSize(#blks)

i-NodeTableLength(#blks)

RootDirectory(i-Node#)

The figure below shows the different in-memory data structures and how they connect to the other

components. We need at least a table that combines the open file descriptor tables (the per-process one and

system-wide one) in a UNIX-like operating system. We simplify the situation because we assume that only

one process is accessing a file at any given time.

When a file is opened, we create an entry in this table. The index of the newly created entry is the “file

descriptor” that is returned by the file opening activity. That is the return value of the sfs_fopen() is this

index. The entry created in the open file descriptor table has at least three pieces of important information:

i-Node number, read pointer, and write pointer. The i-Node number is the one that corresponds to the file.

Remember just like there is an i-Node for the root directory, there is one i-Node associated with each file.

When a file is opened that i-Node is number is recorded in this table entry. The read and write pointers are

also set as specified earlier.

The in-memory data structures are activated as soon as the file system is up and running and they are

updated every time a file system operation is carried out. While designing and implementing a given file

system operation you need to think of the actions that should be carried out on the in-memory and on-disk

data structures. In addition to the Open File Descriptor Table, we have variety of different caches for iNodes,

disk blocks and the root directory. Your design could implement all of them or some of them. File

system performance is not a concern for this assignment – correct operation is what we need.

Rough pseudo code for creating a file:

1. Allocate and initialize an i-Node. You need to somehow remember the state of the i-Node table to

know which i-Node could be allocated for the newly created file. Simply remembering the last iNode

used is not correct because as you delete files, some i-Nodes in the middle of the table will

become unused and available for reuse.

2. Write the mapping between the i-Node and file name in the root directory. You could simply update

the memory and disk copies.

3. No disk data block allocated at file creation. That is, the file size is set to 0.

4. This can also “open” the file for transactions (read and write). Note that the SFS API does not have

a separate create() call. So you can do this activity as part of the open() call.

Rough pseudo code for writing to a file:

1. Allocate disk blocks (mark them as allocated in your free block list).

2. Modify the file's i-Node to point to these blocks.

3. Write the data the user gives to these blocks.

4. Flush all modifications to disk.

5. Note that all writes to disk are at block sizes. If you are writing few bytes into a file, this might

actually end up writing a block. So, if you are writing to an existing file, it is important you read the

last block and set the write pointer to the end of file. The bytes you want to write go to the end of

the bytes that are already part of the file. After you have written the bytes, you flush the block(s) to

the disk.

Rough pseudo code to seek on a file:

1. Modify the read and write pointers in memory. There is nothing to be done on disk!

4. What to Hand In

We have given you a Makefile, disk emulator (C and Header), SFS test files, and FUSE wrappers. The

Makefile shown below has three configurations. The first two use hand coded test files to test your

implementation. Getting your implementing running with these two test files will get you a maximum of

85% grade. If you get the FUSE part working, you get the full 100% provided it does not crash!

CFLAGS = -c -g -Wall -std=gnu99 `pkg-config fuse --cflags --libs`

LDFLAGS = `pkg-config fuse --cflags --libs`

# Uncomment one of the following lines to run the corresponding scenario

#SOURCES= disk_emu.c sfs_api.c sfs_test.c sfs_api.h

#SOURCES= disk_emu.c sfs_api.c sfs_test2.c sfs_api.h

SOURCES= disk_emu.c sfs_api.c fuse_wrappers.c sfs_api.h

OBJECTS=$(SOURCES:.c=.o)

EXECUTABLE=First_Lastname_sfs

all: $(SOURCES) $(HEADERS) $(EXECUTABLE)

$(EXECUTABLE): $(OBJECTS)

gcc $(OBJECTS) $(LDFLAGS) -o $@

.c.o:

gcc $(CFLAGS) $< -o $@

clean:

rm -rf *.o *~ $(EXECUTABLE)