1. CIT 470: Advanced Network and System Administration
Filesystems I
CIT 470: Advanced Network and System Administration

2. CIT 470: Advanced Network and System Administration
Topics
Filesystems and Namespaces
Filesystem Types
Inodes and Superblocks
Network Filesystems
CIT 470: Advanced Network and System Administration

3. CIT 470: Advanced Network and System Administration
Filesystems
A filesystem is a method for storing and organizing documents.
Most filesystems offer a hierarchical tree structure of folders within folders.
Some filesystems are flat, with no folders.
Some filesystems work like a database, where files are identified by metadata, such as creator or user-created tags.
CIT 470: Advanced Network and System Administration

4. CIT 470: Advanced Network and System Administration
Kernel Storage Layers
CIT 470: Advanced Network and System Administration

5. Filesystem Tree Structure/
bin
boot
tmp
usr
var
bin
lib
X11R6 ls
grub
less
vmlinuz
bin
lib
zip
menu.lst
xclock
xterm
CIT 470: Advanced Network and System Administration

6. CIT 470: Advanced Network and System Administration
UNIX has One Namespace
A single tree-structured namespace which
Provides a single way to identify files by name
Contains multiple filesystems:
/dev – files represent hardware devices
/media/cdrom – ISO9660 optical media filesystem
/proc – in-memory representation of kernel data
that are added to the namespace with the mount command: mount /dev/devname /fs/location
CIT 470: Advanced Network and System Administration

7. Namespace contains many fs
CIT 470: Advanced Network and System Administration

8. Filesystem Types by Media
Disk Filesystems
Filesystems designed to store files to a fixed or removable permanent storage device.
examples: ext4fs, FAT, ISO9660, NTFS
Solid State Filesystems
Wear leveling: re-arrange block usage to avoid writing too many times to any one block on flash.
In-Memory Filesystems
Filesystems that represent kernel data structures, e.g. procfs, devfs.
Network Filesystems
Filesystems where file access operations are performed using network operations to contact a server where the data is stored on a disk or other physical medium.
CIT 470: Advanced Network and System Administration

9. Common Disk-based Filesystems
Extended Filesystems
ext2: first full featured UNIX fs for Linux in 1993
Recommended use: USB + other solid state drives.
ext3: + journaling; 2TB max file size; 16TB max vol
ext4: faster version of ext3 with larger max file + vol size
Microsoft Filesystems
FAT: inefficient disk usage, slow, 8+3 filenames
4GB maximum file size in 32-bit FAT
NTFS: modern filesystem, many versions
Supports long + old 8+3 filenames for compatibility
CIT 470: Advanced Network and System Administration

10. Ext Filesystem Structure
CIT 470: Advanced Network and System Administration

11. Superblocks and Block Groups
CIT 470: Advanced Network and System Administration

12. Inode Block Addressing
CIT 470: Advanced Network and System Administration

13. Journaling Filesystems
Problem: writing to file involves many disk writes
Modify inode to change file size
(potentially) Add new data block to used block map
(potentially) Add pointer to new data block
Write to new data block
Journaling filesystems perform writes by:
Write blocks to journal.
Wait for write to be committed to journal.
Write blocks to filesystem.
Discard blocks from journal.
CIT 470: Advanced Network and System Administration

14. CIT 470: Advanced Network and System Administration
Creating a Filesystem
Select a disk partition to create filesystem on
fdisk –l /dev/sda will list partitions on 1st disk
fdisk –l /dev/sdb will list partitions on 2nd disk,
Run mke2fs –v /dev/sda2
Creates ext2 filesystem on 2nd partition of 1st disk
Wipes any data already existing on that filesystem
Add a –j option to create an ext3 journaling fs.
CIT 470: Advanced Network and System Administration

15. CIT 470: Advanced Network and System Administration
Mounting a Filesystem
Create a mountpoint
mkdir -p /stor/video
Mount filesystem on chosen directory
mount -t ext3 /dev/sda2 /stor/video
Use filesystem
Unmount filesystem when done
umount /dev/sda2
Happens automatically at reboot or shutdown
CIT 470: Advanced Network and System Administration

16. CIT 470: Advanced Network and System Administration
Automatic Mounting
Filesystems in /etc/fstab are mounted on boot.
Use mount to see current mounted filesystems.
# /etc/fstab: static file system information. #
# <device> <mnt pt> <type> <options> <dump> <pass>
proc /proc proc defaults
/dev/sda1 / ext3 defaults
/dev/sda2 none swap sw
/dev/sda3 /home ext3 defaults
/dev/sdb1 /backup ext3 defaults
CIT 470: Advanced Network and System Administration

17. Checking Filesystem Integrity
fsck utility performs consistency checks
Are used blocks actually used?
Do inodes point to any unused blocks?
Are used inodes pointed to by directory entries?
and repairs inconsistencies if
Sysadmin enters ‘y’ in interactive mode.
Sysadmin uses ‘-y’ argument to do all repairs.
Run fsck with unmounted partition as arg:
fsck –y /dev/sda2
CIT 470: Advanced Network and System Administration

18. CIT 470: Advanced Network and System Administration
Access Control
Read--You can read the file with cat, more, etc.
Write--You can modify the file with vi,
Execute--You can run the file if it’s a program.
CIT 470: Advanced Network and System Administration

19. CIT 470: Advanced Network and System Administration
POSIX ACLs
Specify individual groups and users.
Basic ACL user/group refers to owner.
POSIX ACLs allow specifying users + groups.
To add/modify permissions for a user:
setfacl –m u:username:rw- filename
To add/modify permissions for a group:
setfacl –m g:groupname:rw- filename
CIT 470: Advanced Network and System Administration

20. CIT 470: Advanced Network and System Administration
File Attributes
Attributes extend file permissions:
a: append-only (only root can set)
i: immutable (read-only, only root can set)
s: safe-delete (overwrite, not supported yet)
Use lsattr to view attributes.
Most files do not have any attributes set.
Use chattr to set attributes.
chattr +i /boot/vmlinuz*
CIT 470: Advanced Network and System Administration

21. CIT 470: Advanced Network and System Administration
Network Filesystems
Use filesystem to transparently share files.
Examples:
NFSv3
CIFS
AFS
NFSv4
GoogleFS
CIT 470: Advanced Network and System Administration

22. CIT 470: Advanced Network and System Administration
NFS v3
Network File System
Transparent, behaves like a regular UNIX filesystem.
Uses UNIX UIDs,GIDs,perms but can work on Win.
Since NFS is stateless, file locking and recovery are handled by rpc.lockd and rpc.statd daemons.
Security
Server only lets certain IP addresses mount filesystems.
Client UIDs have same permissions on server as client.
Client root UID is mapped to nobody, but
Root can su to any client UID to access any file.
CIT 470: Advanced Network and System Administration

23. CIT 470: Advanced Network and System Administration
How NFS Works
CIT 470: Advanced Network and System Administration

24. CIT 470: Advanced Network and System Administration
CIFS
Microsoft Network Filesystem
Derived from 1980s IBM SMB net filesystem.
Originally ran over NetBIOS, not TCP/IP.
\\svr\share\path Universal Naming Convention
Auth: NTLM (insecure), NTLMv2, Kerberos
Implementation
MS Windows-centric (filenames, ACLs, EOLs)
Samba: UNIX client and server software.
CIT 470: Advanced Network and System Administration

25. CIT 470: Advanced Network and System Administration
AFS
Distributed filesystem
Global namespace: /afs/abc.com/vol_home1
Servers provide one or more volumes.
Volume replication with RO copies on other svrs.
Cells are administrative domains within AFS.
Cells contain multiple servers.
Each server provides multiple volumes.
Security
Kerberos authentication
ACLs with user-controlled groups
CIT 470: Advanced Network and System Administration

26. CIT 470: Advanced Network and System Administration
NFSv4
New model of NFS
Only one protocol (no separate mount,lock,etc.)
Global namespace.
Security (ACLs, Kerberos, encryption)
Cross platform + internationalized.
Better caching via delegation of files to clients.
CIT 470: Advanced Network and System Administration

27. CIT 470: Advanced Network and System Administration
References
Michael D. Bauer, Linux Server Security, 2nd edition, O’Reilly, 2005.
Mike Eisler, Ricardo Labiaga, Hal Stern, Managing NFS and NIS, 2nd edition, O’Reilly, 2001.
Aeleen Frisch, Essential System Administration, 3rd edition, O’Reilly, 2002.
Evi Nemeth et al, UNIX System Administration Handbook, 3rd edition, Prentice Hall, 2001.
NFS HOWTO,
RedHat, Red Hat Enterprise Linux 4 System Administration Guide,
RedHat, Red Hat Enterprise Linux 4 Reference Guide,
CIT 470: Advanced Network and System Administration