Project 5 Roadmap
Logical disk layout
Directory Entry
GOSFS_Dir_Entry
blockList[10]
flags
MAX_FILES_PER_DIR
size
filename[128]
acl[10]
data blocks
GOSFS_Dir_Entry
GOSFS_Dir_Entry
GOSFS_Dir_Entry
GOSFS_Dir_Entry
GOSFS_Dir_Entry
GOSFS_Dir_Entry
GOSFS_Dir_Entry
Directory Block
Directory
Data storage –Direct
Data storage – Direct Mapping
Mapping
GOSFS_Dir_Entry.blockList[10]
0
1
2
3
4
5
6
7
8
9
12KB file depicted
DISK
4KB data block
4KB data block
4KB data block
= used
Data storage
Indirect
Data storage –Single
– Single Indirect
GOSFS_Dir_Entry.blockList[10]
40KB file depicted
4KB data block
….
0
1
2
3
4
5
6
7
8
9
DISK
4KB data block
4KB data block
single indirect block
0
1
2
3
4
.
.
.
1022
1023
4KB data block
4KB data block
Data storage –Double
Data storage – Double Indirect
Indirect
GOSFS_Dir_Entry.blockList[10]
4136KB file depicted
4KB data block
single indirect block
0
1
2
3
4
.
.
.
1022
1023
….
0
1
2
3
4
5
6
7
8
9
DISK
4KB data block
4KB data block
0
1
2
3
4
.
.
.
1022
1023
double indirect block
0
1
2
3
4
.
.
.
1022
1023
single indirect block
4KB data block
4KB data block
Data storage - overview
[0,32KB] – use direct mapped blocks
only
 (32KB,4128KB] – use direct mapped
blocks + single indirect blocks
 (4128KB,32MB] – use direct mapped
blocks + single indirect blocks + double
indirect blocks

Format
 SYS_FORMAT

Format(char *dev, char *fstype)
 Original
Disk Layout
 Don’t have to do it in init, will be called
from user space
 Only for GOSFS, not for PFAT
Mount

SYS_MOUNT



Steps




vsf.c: Mount(char *devname, char *pathPrefix, char *fstype)
gosfs.c:GOSFS_Mount(struct Mount_Point *mountPoint)
check superblock validity
fill out mountPoint
might consider using an instance
(see PFAT_Instance in pfat.c)
Make sure it works, so that we can test the
rest of your project
Syscalls

SYS_OPEN

Open(char *name, int mode)

Not allowed for directories
Create files when appropriate



SYS_OPENDIRECTORY

OpenDirectory(char *path)

Not allowed for files
SYS_CLOSE


Close(int fd)
SYS_DELETE

Delete(char *path)

If path is a non-empty directory and not empty, forbid !
Syscalls

SYS_READ

Read(int fd, char *buffer, int length)

Not allowed for directories
return value
 <0, if failure
 >=0 , # bytes available


SYS_WRITE

Write(int fd, char *buffer, int length)

Not allowed for directories
“Grow on write” - allocate blocks “on the fly” if past end of file
“Grow minimum needed” – a random seek to the middle of an
empty file and a write result in only one allocated block
Synchronous




SYS_READ/SYS_WRITE increase the offset with #bytes
read/written
Syscalls

SYS_READENTRY

ReadEntry(int fd,VFS_Dir_Entry *entry)

Read and return information about a single file in a
directory
Advance one file entry at a time, return –1 when
the directory ends
Use File->filePos to remember current entry
Seek specifies number of entries to skip (i.e.
seek(5) would then read the 5th entry)
Not allowed for files




Syscalls

SYS_STAT

Stat(char *path, VFS_File_Stat *stat)

Works on an unopened file
Simply copy verbatim from GOSFS_Dir_Entry


SYS_FSTAT

FStat(int fd, VFS_File_Stat *stat)

Works on an open file
Simply copy verbatim from GOSFS_Dir_Entry


SYS_SEEK

Seek(int fd, int offset)

Implement only absolute semantics for offset
Allowed to past end of file

Syscalls

SYS_CREATEDIR

CreateDirectory(char *name)

Should NOT go recursively


e.g. CreateDirectory("/d/d1/d2/d3/d4”);
SYS_SYNC

Sync(void)

Should synchronize any outstanding data to disk
The Buffer Cache
idea: keep some disk blocks in memory
(buffers); memory is much faster to
read/write than disk
 buffer life cycle



Get - Modify - Release
bufcache.c





Create_FS_Buffer_Cache(struct Block_Device *dev,…)
Get_FS_Buffer()
Modify_FS_Buffer()
Sync_FS_Buffer()
Release_FS_Buffer()