File systems and Filesystem quota
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1 File systems and Filesystem quota 8.1
2 Unit objectives After completing this unit, you should be able to: Describe what a file is Describe what a file system is List possible file systems Describe i-nodes Create/mount/unmount file systems Create predefined mounts Set up user and group quota
3 What is a file? Consecutive number of bytes No internal structure by default (applications define their own structure) Stored and referenced in a file system Can have multiple references (names) Special files exist Block, Character > Device Pipes, Sockets > Interprocess communication
4 What is a file system? Place to store files and refer to them Hierarchical structure through use of directories A file system can be stored on any block device Floppy disk Hard disk Partition RAID, LVM volume File (for use with a loop device) RAM disk /bin /dev /etc /lib /sbin /usr /var
5 The virtual file system User processes vi, ls, mv, rm, file, strings, cat, touch... System call interface open() read() write() close()... VFS abstraction layer ext2 reiserfs minix ext3... Buffer cache Device drivers I/O request Hardware
6 File systems supported Traditional: ext2 Second generation: ext3, ReiserFS, IBM JFS, xfs Next generation: ext4, GFS2, Reiser4 FAT-12, FAT-16, FAT-32, VFAT, NTFS (read-only) CD-ROM (ISO 9660) UMSDOS (UNIX-like FS on MS-DOS) NFS (Network File System) SMBFS (Windows share), NCPFS (Novell Netware share) /proc (for kernel and process information) SHMFS (Shared Memory File System) GPFS, Lustre (Clustering File Systems)
7 Basic file system example: ext2 Partition divided into blocks of 1024, 2048, or 4096 bytes Block size depends on size of file system and expected usage Blocks can have different usage: Superblock Index node (i-node) block Indirect block (double, triple) Data block S I I D D S I I ID D D D
8 Superblock First block of file system, several copies (at 8193, 16385,...) Contains general info on file system Last mounted time/place Block size Pointers to free i-nodes Pointers to free blocks Pointer to root of file system Use dumpe2fs to view filesystem/superblock info S I I D D S I I ID D D D
9 i-nodes 128 bytes (8 per block of 1024 bytes) Contains information about a file: owner, group, type, size, permissions, ctime, atime, mtime,... Contains pointers to data blocks Contains pointers to an indirect block, a double indirect block, and a triple indirect block Use stat command to view inode info Owner/=gGroup File type File size File permissions Time stamps: create time access time modification time Link counter Additional flags: (ACL, EXT2,_FLAGS) Pointers to block data S I I D D S I I ID D D D
10 Data blocks Contain file data File can be a directory, in which case the data is the list of file names and i-nodes in that directory. Multiple file names can point to the same i-node! (In other words, a file can have multiple names.) i-node 3694 Data 6417 i-node 8391 Data 9041 Type: d Data: 6417 Size: 1024 User: 0 Group: 0 Name... xyz abc i-node Type: f Data: 9041 Size: 21 User: 0 Group: 0 Link: 2 File data xyz Directory Regular file S I I D D S I I ID D D D
11 Ext2fs summary The most important components of a file system are the i- nodes and the data blocks. The file system is full if: No more i-nodes are available No more data blocks are available So tune your file system according to the number of bytes per file: Blocksize (1024, 2048, or 4096 possible) Bytes per i-node (4096 default)
12 Other file system features File systems can have other features that can be useful: Access Control Lists (ACL) Allow more extended permissions, not just rwxrwxrwx The most commonly used file systems have ACL support Journaling Keeps a journal of operations that are going to take place and operations that were successfully committed Should make recovery from a crash faster Slight performance decrease Most modern file systems use journaling to some degree. Extended file attributes Examples: immutable, auto compression, undeletable Labels Allow mounting based on label instead of device name Performance optimizations
13 Creating a file system Creating a file system is done with an mkfs variant. mke2fs, mke2fs j, mkfs -t mkreiserfs mkjfs Typical options: b blocksize sets blocksize i bytes per i-node sets number of i-nodes c checks disk for bad blocks Example: # mke2fs -b i c /dev/sda6... Writing inode tables: done Writing superblocks and filesystem accounting info: done...
14 Mounting a file system Using the mount command: Supply device name Supply mount point (empty directory) # mount -t ext3 /dev/sda6 /mnt/extra # mount -o nodev,noexec /dev/system/mylv /usr/local/proj1 Optional: Supply file system type Optional: Supply other options Optional: Use different superblock To show mounted file systems, use mount without arguments.
15 Mounting file systems at system startup Add to /etc/fstab: /dev/sda1 /boot ext3 defaults 1 2 /dev/sda5 / ext3 defaults 1 1 /dev/cdrom /mnt/cdrom iso9660 noauto,ro,user 0 0 /dev/fd0 /mnt/floppy msdos noauto,user 0 0 /dev/sda6 /mnt/extra ext3 defaults 0 0 Alternative notation, using ext2/ext3 file system labels: LABEL=/boot /boot ext3 defaults 1 2 LABEL=/ / ext3 defaults 1 1 /dev/cdrom /mnt/cdrom iso9660 noauto,ro,user 0 0 /dev/fd0 /mnt/floppy msdos noauto,user 0 0 LABEL=extra /mnt/extra ext3 defaults 0 0
16 Mount options Various options can be used when mounting a file system: auto: Mount file system automatically when booting. noauto: Do not mount fs automatically. user: Users are allowed to mount this file system. owner: Same as user but user must be owner of device ro: Read-only rw: Read-Write For more options, see man mount.
17 Unmounting file systems File system might not be in use: Check with fuser. Open files Programs being executed Active directories # fuser -v /usr USER PID ACCESS COMMAND /usr root Kernel mount /usr Use the umount command with either: The device name The mount point Or both # umount /dev/cdrom # umount /mnt/cdrom
18 Checking a file system Checking a file system is done automatically when the system boots. If a file system is cleanly unmounted, no further checks are done. Minor errors repaired automatically. Major errors drop you in a shell; allows you to do a more thorough check manually. fsck -y /dev/sda1 Can start file system checks manually as well with fsck. Only on file systems that are mounted read-only or not mounted at all
19 ext2/3/4-specific information ext3/4 adds journaling to ext2 using a special, hidden.journal file of arbitrary size (recommended: 10 MB) Thus, downwards compatible with ext2 For new ext3 file systems, use mke2fs -j For converting ext2 to ext3, use tune2fs -j ext4 adds performance optimizations to ext2/3 for large files Useful ext2/3/4 commands: tune2fs tunes an ext2/3/4 file system debugfs debugs an ext2/3/4 file system chattr changes ext2/3/4 extended attributes of a file Immutable, Compressed, Undeletable and so forth (see man chattr for details) e2label changes file system label of an ext2/3/4 file system resize2fs can increase the size of a mounted file system and decrease the size of an unmounted file system
20 ReiserFS-specific information File system for Linux only, created by Hans Reiser 32 MB journal by default (minimum 2 MB) Thus, do not use ReiserFS for small file systems. Journal can be in the file system itself or on a separate partition. Uses balanced trees instead of linear directory lists. Extremely useful for directories that contain files Useful commands: debugreiserfs debugs a ReiserFS file system. resize_reiserfs resizes a ReiserFS file system. Extending can be done on a mounted file system. Reducing can only be done on an unmounted file system. reiserfsck runs a file system check on a ReiserFS. Due to legal issues, no longer used as the default file system type in modern distributions.
21 Comparing file systems Journaled file systems used by Linux: ext2 ext3 ext4 jfs reiser Journal No Yes (10 MB default) Yes (10 MB default) Yes (auto resized) Yes (32 MB default) Resizeable Yes, but only when unmounted Yes Yes Yes Yes Maximum size File: 2 TB FS: 16 TB File: 2 TB FS: 16 TB File: 16 TB FS: 1 EB File: 4 PB FS: 32 PB File: 16 TB FS: 1 EB Type i-nodes (completely block oriented) i-nodes (completely block oriented) i-nodes (blocks and extents) i-nodes (allocated in a b-tree) b-tree
22 SHMFS-specific information SHMFS: POSIX compliant Shared Memory File system File system stored in memory, expands when used to required size Not persistent across reboot Typically mounted on /dev/shm Required by certain applications
23 Quota concepts Quotas limit the amount of data a user/group is allowed to store. Defined on a per file system basis Based on block and/or i-node usage per user or group Two limits per quota: Soft and hard User exceeds soft limit warning only User exceeds hard limit error Grace period identifies how long the soft limit can be exceeded. After that period, a user gets errors instead of warnings. 20 MB 5 MB File system: 300 MB Each user can consume only 20 MB permanently and 25 MB temporarily.
24 Quota implementation on Linux Quota support compiled into the kernel No daemon necessary Implemented on a per file system basis A user can have different quota on different file systems. Stored in aquota.user and aquota.groups in the root of the file system Quota checking should be enabled when mounting the file system. Mount options: usrquota, grpquota Can be specified in /etc/fstab Quota checking should be turned on after mounting with the quotaon command. Automatically executed from bootscript after mount -a
25 Enabling quota Modify /etc/fstab /dev/sda2 / ext3 defaults 1 1 /dev/sda4 /home ext3 defaults,usrquota,grpquota 1 2 /dev/sdb /mnt/cdrom iso9660 noauto,owner,ro 0 0 /dev/sda3 swap swap defaults 0 0 /dev/fd0 /mnt/floppy msdos noauto,owner 0 0 none /proc proc defaults 0 0 none /dev/pts devpts gid=5,mode= Create aquota.user and aquota.group in the file system's root directory. Remount the partition. Calculate current usage and turn on quota checking. # touch /home/aquota.user /home/aquota.group # mount -o remount /home # quotacheck /home # quotaon /home
26 Configuring quota Done with the edquota command Starts $EDITOR (default: vi) in a subshell. Only edit the block/i-node soft/hard quota number. User quota: edquota -u username Disk quotas for user tux1 (uid 501): Filesystem blocks soft hard inodes soft hard /dev/sda /dev/sda ~ ~ ~ "/tmp/edp.a9fseqk" 3L, 213C Group quota: edquota -g groupname Grace period: edquota -t Copy quota: edquota -p tux1 -u tux2 tux3 tux4
27 Quota information quota command Reports on the quota of one user Can be executed by anyone A regular user can only view his own quota tux1$ quota Disk quotas for user tux1 (uid 501): Filesystem blocks quota limit grace files quota limit grace /dev/sda repquota command Reports on the quota of all users and groups Can only be executed by root root# repquota /dev/hda4 Block limits File limits User used soft hard grace used soft hard grace root tux days none tux
28 Checkpoint 1. Assuming a blocksize of 1024, how many i-nodes and data blocks do you need for a file on an ext2 file system? a. With size 0? b. With size 1? c. With size 2000? d. With size (12 K+1)? 2. What are the two methods of copying a file to a (not yet mounted) MS-DOS floppy? 3. What files are important with respect to quotas?
29 Checkpoint solutions 1. Assuming a blocksize of 1024, how many i-nodes and data blocks do you need for a file on an ext2 file system? a. With size 0? The answer is 1 i-node and 0 data blocks. b. With size 1? The answer is 1 i-node and 1 data block. c. With size 2000? The answer is 1 i-node and 2 data blocks. d. With size (12 K+1)? The answer is1 i-node and 12 data blocks directly from the i-node, an indirect block, and an extra data block. Total 14 data blocks. 2. What are the two methods of copying a file to a (not yet mounted) MS-DOS floppy? The answer is mount file system, and use cp command and use mcopy (from mtools). 3. What files are important with respect to quotas? The answer is /etc/fstab to specify file systems and /quota.users and /quota.groups.
30 Exercise: File systems and file system quota What you will do in this exercise: Create and manipulate file systems Configure user quota
31 Unit summary Having completed this unit, you should be able to: Describe what a file is Describe what a file system is List possible file systems Describe i-nodes Create/mount/unmount file systems Create predefined mounts Set up user and group quota
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