On Fri, 29 Mar 2013 15:44:49 +0530, Sankar P said:
>> I have decided on a simple layout for my filesystem where the first
>> block will be the super block and will contain the version
>> information etc. The second block will contain the list of inodes.
>> Third block onwards will be data blocks. Each file can grow only up to
>> a single block size. Thrid block will represent the first file, fourth
>> block for the second file and so on. Directories will not be
>> supported.
You *will* have to support at least the top-level directory, because
you'll need at least directory entries for "." and "..". If your second
block is "list of inodes", then you have directories (and adding subdir
support isn't *that* hard).
You'll also want either a list or bitmap structure or some other way to
determine if a given block is allocated - trying to write a new file without
having a freelist to get blocks from is hard. Oh, and don't forget to
add locking around the freelist operations and similar things - having
two processes both grab block 27 for the file they just created can suck :)
> oh okay. But how do I create the superblock ? What are the APIs
> available to do these block level operations from a user space
> application (my mkfs program ) ?
struct foobar_suoper {
int version;
int num_files;
int free_blocks;
char padding[512-3*sizeof(int)];
};
struct foobar_super sb;
int disk;
bzero(sb, sizeof struct foobar_super);
sb.version = 1;
sb.num_files= 0;
sb.free_blocks = 999; /* should probably set to actual size of partition/file */
disk = open(*diskorfilename, ....); /* testing on loop mounts is useful */
lseek ( disk, 0);
write (disk, &sb, sizeof(sb)); /* congrats, you just wrote a superblock */
Yes, it's that simple :) You want to write some empty inodes, add a
'struct inode' variable, initialize it, lseek to were the inode goes and
write it out.
Just open, lseek, write, close. ;) And yes, those operations *do* work
just fine on both files you then use woth 'mount -o loop' and with /dev/sd*
or /dev/mapper/* LVM.
You might want to look at the source for mkfs.vfat (part of dosfstools package)
for additional details.
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