Baby Cow File System - Spec 1

Baby Cow File System

Spec 1

2005/09/05

Design principles:

1.         Assume CHS are 24 bits long, so each link is 3 bytes long, only max 8 GB hard disk is supported. Really don’t know how to control a larger hard disk by using port : (

2.         Stable is the most important, second is speed. So my file system is atomic, either the operation is fully complete, or it is not. There is an atomic field, which is 4096 bytes long, when you write anything to the partition, backup the original block to that field first. !!! This will highly decrease the performance because write 1 block, it performs 2 write operations. !!! ReiserFS said they do it the same way, but they have better algorithm to fix this performance problem, if anyone know it, please tell me : )

3.         My file system is not journal, but it is safety enough for single read/write operation.

4.         I will not put all the field too tight, that mean even the field is 1 bit width, I will make it occupies 1 byte because I don’t want to perform &/| operation to retrieve the byte, it waste the CPU clock.

 

Feature will be added late:

1.           Map a directory of file to a memory

2.           Better permission format, more suitable to my operating system.

 

Baby Cow file system specification

Number of file within a directory

4294967296

Number of sub directory

4294967296

Maximum file size

2^64 / 16777216 TB

Maximum filename

500 single byte character

Maximum directory name

500 single byte character

Maximum partition size

2^42-4*Block size – 4096 Bytes

Block size

Any but must larger than 4096 Bytes

 

 

 

Hard disk layout

Super block

Atomic field

Any other block

4096 Bytes

4096 Bytes

Excess bytes in the partition

 

 

Super Block: fixed 4096 Bytes long, used to store the information of the partition. It must be located in the first 4096 bytes of the partition.

Byte Offset

0

2

502

510

518

526

528

536

Name

ID

Partition Name

Root directory link

Create time

Last modified time

Block Size in KB

Number of Free address Block

Unused

Width

2

500

8

8

8

2

8

3560

Example value

‘S’,’B’

“partition name”

LBA

Second since AD 2000

Second since AD 2000

4 (for 4096KB block size)

4 (for 4 blocks)

-

Descriptions:

1.         Root directory list link, link to the block of root directory of partition, 24bit CHS value.

 

Free Address Block: there can be many free address list block, depend on the size of the partition, but these blocks must be following the super block.

Byte Offset

0

3

Name

ID

Free Block List

Width

3

Block size - 3

Example value

‘F’,’A’,’B’

Figure 1

 

Byte Offset

0

Width

1

Example value

0 : free

1 : used

(Figure 1)

 

Number of Block = (Partition Size – 4096(Super Block size)) / Block Size

Free Address Block Capacity = (Block Size – 3 bytes) blocks. One Free Address Block can arrange up to (Block Size – 3 bytes) number of block.

Number of Free Address Block = Free Address Block Capacity / Number of Block

 

Directory Block : used to store the information of the directory and the links to its sub-directory.

Byte Offset

0

3

503

507

511

520

528

536

544

Name

ID

Directory Name

Number of File

Number of Directory

Permission

Create time

Last modifed time

Directory indirect block

File or Directory links

Width

3

500

4

4

9

8

8

8

Block size - 544

Example value

‘D’,’I’,’R’

“/root/password”

52

64

rwx-rw-rw

Second since AD 2000

Second since AD 2000

Block ID

Figure 2

Descriptions:

1.         Sub-directory/file link : each 4-bytes represent as follow:

ID

LBA

0 (File)

Block ID

1 (Directory)

Block ID

              (Figure 2)

Number of link per directory block = (Block size – 542) / 8

 

Directory Indirect Block

Byte Offset

0

3

Name

ID

File/Directory links

Width

3

Block size - 542

Example value

‘D’,’I’,’B’

Figure 2

 

Number of link per directory indirect block = (Block size -3) / 8

 

File Block : used to store the information and content of the file.

Byte Offset

0

4

504

513

521

529

537

545

Name

ID

Filename

Permission

Create time

Last modifed time

Filesize

File indirect block link

File Content Block

Width

4

500

9

8

8

8

8

Block size - 537

Example value

‘F’,’I’,’L’,’E’

“/root/a/b.txt”

rwx-rw-rw

Second since AD 2000

Second since AD 2000

20

Block ID

Block ID

 

File Content Block : used to store the information and content of the file.

Byte Offset

0

Name

File Content

Width

Block size

Example value

-

 

File Indirect Block : used to store the link, which pointed to File Content Block

Byte Offset

0

3

Block size - 8 - 3

Name

ID

File Content Block Link

File Indirect Block Link

Width

FIB

Block Size – 8 - 3

8

Example value

‘F’,’I’,’B’

-

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