Re: Designing Another File System

[John Richard Moser]

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Phil Lougher wrote:
| On Mon, 29 Nov 2004 23:32:05 -0500, John Richard Moser
| <nigelenki@xxxxxxxxxxx> wrote:
|
|
|>- - localization of Inodes and related meta-data to prevent disk thrashing
|
|
| All filesystems place their filesystem metadata inside the inodes.  If
| you mean file metadata then please be more precise.  This isn't
| terribly new, recent posts have discussed how moving eas/acls inside
| the inode for ext3 has sped up performance.
|

I got the idea from FFS and its derivatives (UFS, UFS2, EXT2).  I don't
want to store xattrs inside inodes though; I want them in the same block
with the inode.  A few mS for the seek, but eh, the data's right there,
not on the other side of the disk.

|
|>- - a scheme which allows Inodes to be dynamically allocated and
|>deallocated out of order
|>
|
|
| Um,  all filesystems do that, I think you're missing words to the
| effect "without any performance loss or block fragmentation" !

All filesystems allow you to create the FS with 1 inode total?


Filesystem            Inodes   IUsed   IFree IUse% Mounted on
/dev/hda1            7823616  272670 7550946    4% /

No, it looks like this one allocates many inodes and uses them as it
goes.  Reiser has 0 inodes . . .

|
|
|>- - 64 bit indices indicating the exact physical location on disk of
|>Inodes, giving a O(1) seek to the Inode itself
|
|
|>1)  Can Unix utilities in general deal with 64 bit Inodes?  (Most
|>programs I assume won't care; ls -i and df -i might have trouble)
|>
|
|
| There seems to be some confusion here.  The filesystem can use 64 bit
| inode numbers internally but hide these 64 bits and instead present
| munged 32 bit numbers to Linux.
|
| The 64 bit resolution is only necessary within the filesystem dentry
| lookup function to go from a directory name entry to the physical
| inode location on disk.  The inode number can then be reduced to 32
| bits for 'presentation' to the VFS.  AFAIK as all file access is
| through the dentry cache this is sufficient.  The only problems are
| that VFS iget() needs to be replaced with a filesystem specific iget.
| A number of filesystems do this.  Squashfs internally uses 37 bit
| inode numbers and presents them as 32 bit inode numbers in this way.
|

Ugly, but ok.  What happens when i actually have >4G inodes though?

|
|>3)  What basic information do I absolutely *need* in my super block?
|>4)  What basic information do I absolutely *need* in my Inodes? (I'm
|>thinking {type,atime,dtime,ctime,mtime,posix_dac,meta_data_offset,size,\
|>links}
|
|
| Very much depends on your filesystem.  Cramfs is a good example of the
| minimum you need to store to satisfy the Linux VFS.  If you don't care
| what they are almost anything can be invented (uid, gid, mode, atime,
| dtime etc) and set to a useful default.  The *absolute* minimum is
| probably type, file/dir size, and file/dir data location on disk.

I meant basic, not for me.  Basic things a real Unix filesystem needs.
What *I* need comes from my head.  :)

|
|
|>I guess the second would be better?  I can't locate any directories on
|>my drive with >2000 entries *shrug*.  The end key is just the entry
|>{name,inode} pair.
|
|
| I've had people trying to store 500,000 + files in a Squashfs
| directory.  Needless to say with the original directory implementation
| this didn't work terribly well...
|

Ouch.  Someone told me the directory had to be O(1) lookup . . . .

| Phillip Lougher
|

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