BACK TO BASICS Priorities and nice
Unix systems (and thus Linux) are multi-tasking and multi-user systems. Indeed, several processes can run in parallel, and be owned by different users: the kernel mediates access to the resources between the different processes. As a part of this task, it has a concept of priority, which allows it to favor certain processes over others, as needed. When you know that a process can run in low priority, you can indicate so by running it with nice program. The program will then have a smaller share of the CPU, and will have a smaller impact on other running processes. Of course, if no other processes needs to run, the program will not be artificially held back.
nice works with levels of “niceness”: the positive levels (from 1 to 19) progressively lower the priority, while the negative levels (from -1 to -20) will increate it — but only root can use these negative levels. Unless otherwise indicated (see the nice(1) manual page), nice increases the current level by 10.
If you discover that an already running task should have been started with nice it is not too late to fix it; the renice command changes the priority of an already running process, in either direction (but reducing the “niceness” of a process is reserved to the root user).
Installation of the anacron package deactivates execution by cron of the scripts in the /etc/cron.hourly/, /etc/cron.daily/, /etc/cron.weekly/, and /etc/cron.monthly/ directories. This avoids their double execution by anacron and cron. The cron command remains active and will continue to handle the other scheduled tasks (especially those scheduled by users).
9.9. Quotas
The quota system allows limiting disk space allocated to a user or group of users. To set it up, you must have a kernel that supports it (compiled with the CONFIG_QUOTA option) — as is the case of Debian kernels. The quota management software is found in the quota Debian package.
To activate them in a filesystem, you have to indicate the usrquota and grpquota options in /etc/fstab for the user and group quotas, respectively. Rebooting the computer will then update the quotas in the absence of disk activity (a necessary condition for proper accounting of already used disk space).
The edquota user (or edquota -g group) command allows you to change the limits while examining current disk space usage.
GOING FURTHER Defining quotas with a script
The setquota program can be used in a script to automatically change many quotas. Its setquota(8) manual page details the syntax to use.
The quota system allows you to set four limits:
two limits (called “soft” and “hard”) refer to the number of blocks consumed. If the filesystem was created with a block-size of 1 kilobyte, a block contains 1024 bytes from the same file. Unsaturated blocks thus induce losses of disk space. A quota of 100 blocks, which theoretically allows storage of 102,400 bytes, will however be saturated with just 100 files of 500 bytes each, only representing 50,000 bytes in total.
two limits (soft and hard) refer to the number of inodes used. Each file occupies at least one inode to store information about it (permissions, owner, timestamp of last access, etc.). It is thus a limit on the number of user files.
A “soft” limit can be temporarily exceeded; the user will simply be warned that they are exceeding the quota by the warnquota command, which is usually invoked by cron. A “hard” limit can never be exceeded: the system will refuse any operation that will cause a hard quota to be exceeded.
VOCABULARY Blocks and inodes
The filesystem divides the hard drive into blocks — small contiguous areas. The size of these blocks is defined during creation of the filesystem, and generally varies between 1 and 8 kilobytes.
A block can be used either to store the real data of a file, or for meta-data used by the filesystem. Among this meta-data, you will especially find the inodes. An inode uses a block on the hard drive (but this block is not taken into consideration in the block quota, only in the inode quota), and contains both the information on the file to which it corresponds (name, owner, permissions, etc.) and the pointers to the data blocks that are actually used. For very large files that occupy more blocks than it is possible to reference in a single inode, there is an indirect block system; the inode references a list of blocks that do not directly contain data, but another list of blocks.
With the edquota -t command, you can define a maximum authorized “grace period” within which a soft limit may be exceeded. After this period, the software limit will be treated like a hard limit, and the user will have to reduce their disk space usage to within this limit in order to be able to write anything to the hard drive.
GOING FURTHER Setting up a default quota for new users
To automatically setup a quota for new users, you have to configure a template user (with edquota or setquota) and indicate their user name in the QUOTAUSER variable in the /etc/adduser.conf file. This quota configuration will then be automatically applied to each new user created with the adduser command.
9.10. Backup
Making backups is one of the main responsibilities of any administrator, but it is a complex subject, involving powerful tools which are often difficult to master.
Many programs exist, such as amanda, a client/server system featuring many options, whose configuration is rather difficult. BackupPC is also a client/server solution, but with a web interface for configuration which makes it more user-friendly. Dozens of other Debian packages are dedicated to backup solutions, as you can easily confirm with apt-cache search backup.
Rather than detailing some of them, this section will present the thoughts of the Falcot Corp administrators when they defined their backup strategy.
At Falcot Corp, backups have two goals: recovering erroneously deleted files, and quickly restoring any computer (server or desktop) whose hard drive has failed.
9.10.1. Backing Up with rsync
Backups on tape having been deemed too slow and costly, data will be backed up on hard drives on a dedicated server, on which the use of software RAID (see Section 12.1.1, “Software RAID”) will protect the data from hard drive failure. Desktop computers are not backed up individually, but users are advised that their personal account on their department's file server will be backed up. The rsync command (from the package of the same name) is used daily to back up these different servers.
BACK TO BASICS The hard link, a second name for the file
A hard link, as opposed to a symbolic link, can not be differentiated from the linked file. Creating a hard link is essentially the same as giving an existing file a second name. This is why the deletion of a hard link only removes one of the names associated with the file. As long as another name is still assigned to the file, the data therein remain present on the filesystem. It is interesting to note that, unlike a copy, the hard link does not take up additional space on the hard drive.