Transcript filesystem

CIT 470: Advanced Network and
System Administration
Filesystems I
CIT 470: Advanced Network and System Administration
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Topics
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Filesystems and Namespaces
Filesystem Types
Inodes and Superblocks
Network Filesystems
CIT 470: Advanced Network and System Administration
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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
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Kernel Storage Layers
CIT 470: Advanced Network and System Administration
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Filesystem Tree Structure
/
bin
ls
boot
tmp
usr
bin
grub
lib
var
X11R6
less
vmlinuz
menu.lst
bin
zip
xclock
CIT 470: Advanced Network and System Administration
lib
xterm
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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
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Namespace contains many fs
CIT 470: Advanced Network and System Administration
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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.
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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
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Ext Filesystem Structure
CIT 470: Advanced Network and System Administration
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Superblocks and Block Groups
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Inode Block Addressing
CIT 470: Advanced Network and System Administration
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Journaling Filesystems
Problem: writing to file involves many disk writes
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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:
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Write blocks to journal.
Wait for write to be committed to journal.
Write blocks to filesystem.
Discard blocks from journal.
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Creating a Filesystem
Select a disk partition to create filesystem on
fdisk –l /dev/sda
fdisk –l /dev/sdb
will list partitions on 1st disk
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.
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Mounting a Filesystem
1. Create a mountpoint
mkdir -p /stor/video
2. Mount filesystem on chosen directory
mount -t ext3 /dev/sda2 /stor/video
3. Use filesystem
4. Unmount filesystem when done
umount /dev/sda2
Happens automatically at reboot or shutdown
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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 0
0
/dev/sda1 /
ext3
defaults 0
1
/dev/sda2 none
swap
sw
0
0
/dev/sda3 /home
ext3
defaults 0
1
/dev/sdb1 /backup ext3
defaults 0
0
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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
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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.
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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:rwfilename
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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*
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Network Filesystems
Use filesystem to transparently share files.
Examples:
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NFSv3
CIFS
AFS
NFSv4
GoogleFS
CIT 470: Advanced Network and System Administration
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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
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How NFS Works
http://www.cs.ucla.edu/~kohler/class/05f-osp/notes/lec18.html
CIT 470: Advanced Network and System Administration
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CIFS
Microsoft Network Filesystem
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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
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MS Windows-centric (filenames, ACLs, EOLs)
Samba: UNIX client and server software.
CIT 470: Advanced Network and System Administration
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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
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NFSv4
New model of NFS
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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.
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References
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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, http://nfs.sourceforge.net/nfs-howto
RedHat, Red Hat Enterprise Linux 4 System Administration Guide,
http://www.redhat.com/docs/manuals/enterprise/RHEL-4-Manual/sysadminguide/, 2005.
RedHat, Red Hat Enterprise Linux 4 Reference Guide,
http://www.redhat.com/docs/manuals/enterprise/RHEL-4-Manual/refguide/ch-nfs.html, 2005.
CIT 470: Advanced Network and System Administration
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