Memory management

Linux Memory Management Total memory available for processes = real memory + paging space - 1MB. First megabyte of real memory is used for kernel program and kernel data -> not for applications. –A bzImage kernel might use more than 1 MB. Rest is used for processes. Pages in real memory will be paged out to disk if necessary. Unused real memory will be used for disk caching. The maximum amount of usable memory (on intel-32) is 4GB (if kernel supports it)

Example: Lightly Loaded System On a lightly loaded system –All processes will fit in real memory –Real memory will be left which will be used to cache data on disk to reduce data access time

Example: Heavily Loaded System On a heavily loaded system –less often used process will be swapped out to disk ( paging space) –Only most often used processes will be in memory Linux uses a very efficient and effective, but non-tunable algorithm to decide whether to give up caching space or to swap out processes if real memory becomes full.

Creating Paging Space We need an empty partition –Create with fdisk –Partition type 82 (Linux swap) Create paging space in that partition mkswap -c /dev/hda3 Activate paging space swapon /dev/hda3 –Add to /etc/fstab Deactivating paging space is done with swapoff –In real time, no reboot necessary. –Only if enough memory is available.

Creating Paging Space Paging can also be done to a file (less efficient) –Create a large file and use it as if it were a partition

Useful Commands top : displays memory, CPU process statistics continuously uptime : displays system uptime + load free : displays memory statistics ps : displays processes sync : flushes the cache to disk xload : graphically displays system load

Backup & Restore

Why Backup? Data is very important –Expensive to create –Can it be re-created Disaster recovery –Hardware failure –Software failure –Damage due to installation or repair –Accidental deletion –Malicious users or break

Why Backup? (Cont’d) Long-term archive System Administration –Transfer of data between systems –Reorganizing file system –Defragmentation –Checkpoint before and after an upgrade

Why Restore? Recover files Compare versions of files Recover system after disaster Restore archived data Shrink file systems

Three Types of Backup System backup –Preserves system directories and files Full backup –Preserves all user data and configuration files Incremental backup –Records changes since last full backup or last incremental backup –very fast, but takes more time to restore –must be used carefully –Needs more tapes

Backup Strategy (1) Normal Working System System Backup -> Full Backup -> Incremental Backup -> Incremental Backup -> Incremental Backup ->... Note: Each organisation has its own special considerations concerning data to take into account.

Backup Strategy (1) In practice you would want to exclude certain directories from the backup process. Some of the directories being –/tmp –/var/tmp –/usr/tmp Not backing up these directories saves space on the archives

Backup Strategy (1) Do not backup /proc even if it exists on your system. The /proc filesystem is not a disk file system but is a way for the kernel to provide you with information about the operating system and system memory. You cannot restore it later.

Backup Strategy (2) Back up everything which has changed since last system backup: –All user data –All changed system files –All changed application files Do not back up: –Unchanged application files –Software quickly loaded from tape

Incremental Backup Scheme Level 0 Backup- Sun (Full dump) Level 9 Backup- Mon (Incremental dump) Level 8 Backup- Tues (Incremental dump) Level 7 Backup- Wed (Incremental dump) Level 6 Backup- Thur (Incremental dump) Level 5 Backup- Fri (Incremental dump) Level 4 Backup- Sat (Incremental dump)

Backup Devices Tape drive CD-ROM Hard disk Diskette drive ZIP drive Network

Backup Commands tar –Widely available –Excellent for transferring data between platforms –Beware of empty directories (-d option) cpio –Widely available –Difficulties with many symbolic links

Backup Commands dd –Not a real backup command –Copies are exact images –Can be used for conversions

tar command Traditional UNIX tape archive command –Performs backup and restore Backup with tar: tar cvf [files | dir] tar cvf /dev/fd0 /home tar cvf /mnt/hda8/filename.tar /home

tar command Restore with tar: tar -xvf device [files|dir] tar -xvf /dev/fd0 "/home/ j*" List contents of a tar backup: tar -tvf

cpio Command Common UNIX backup command Backup with cpio: cpio -ov > find /home | cpio -ov > /dev /fd0

cpio Command Restore with cpio: cpio -iv[-dum] [files] cpio -ivdum " /home/ j*" < /dev/fd0 List contents of a cpio backups: cpio -itv cpio itv < /dev/fd0

dd Command Not a real backup command Converts and copies files –dd reads the infile parameter, does the specified conversions –Copies the converted data to the outfile parameter

dd Command Examples: –Makes a backup of /etc/passwd dd if = /etc/passwd of = /dev /rfd0 –Displays the long list of the current directory in uppercase ls -l | dd conv = ucase