An introduction to File System for Unix and Windows

1. File SystemsFile Systems
David MartínezDavid Martínez

2. Content
 Physically, what is a HD like?
 The HD for the OS
 Why do I need a FS?
 FS structure in Windows
 FS structure in Unix
 Other FS
 Journalling

3. Physically, what is a HD like?
 Little exercise
(Floppy)
 2 heads
 80 cylinders
 18 sectors
 512 bytes/sector

4. The HD for the OS
 MBR – Master Boot Record
 Bootloader
 Partition table
 Partitions* (limited to 4 primary)
 File System: many, not only for HDs
 Clusters/Blocks (performance)
 Driver to transform physical address to logical

5. Why do I need a FS?
 To set a structure for the data (files,
directories...)
 Metadata (name, modification date, owner...)
 Set a permissions system
 Data integrity (damaged sectors)
 Links
 The clusters in which a file is saved (they might
not be contiguous)
 ...

6. FS structure in Windows
 Boot Record – contains information about the different
areas
 FAT – File Allocation Table
 One entry for each block in the data area
Boot
Record
FAT
Optional
Duplicate
FAT
Data Blocks
 The FAT family FS

7. FS structure in Unix
 Superblock*: stores the size, number of files, free space,
index of the next free inode...
 i-node list: holds one entry for each file or directory where
to save metadata, inode type, locking and modification
flags...
 Data blocks: keeps the data of the files pointed by the
inodes.
Superblock i-node list Data blocks
 The Ext family FS

8. FS structure in Unix
 The Ext family FS
 Buffer cache
 Syncer
 13 entries per inode
 The first 10 direct
 11º indirect simple
 12º indirect double
 13º indirect triple
 If the block size is 1KB
 Files of 16TB

9. Other FS
 Special FS
 Swap
 ProcFS / SysFS
 DevFS
 TmpFS
 UnionFS
 In Unix everything is a file

10. Journalling
 Avoid corruption
 Write log before commit
 Before journalling
 Guessing work with “fsck”
 For ext3, two ways of mounting the partition
 Async: uses journalling and it's faster
 Sync: old system without journalling, makes
changes straight to the disk

11. Journalling
 Ordered (default)
 Only log of Metadata
 Data written to the disc before writing the log
 Writeback
 Only log of Metadata
 Data written to the disc after or before writing the log
 Journal
 Log of Metadata and Data
 First write the log then the disc
 Slower but the most secure

12. Resources
 http://en.wikipedia.org
 http://users.iafrica.com/c/cq/cquirke/fat.htm
 http://hebb.cis.uoguelph.ca/~dave/CIS275/Lect
ures/file7.html
 Old notes