Disk usage FAQ

Last update: Sat Sep 24 06:47:07 2005     Wed Oct 5 15:51:16 2005               

Table of contents

1. How much disk space do we have?
2. Are disk quotas enforced?
3. Do I have to pay for my disk space?
4. How much disk space can I use?
5. How do I find out how much disk space I am using?
6. How do I find out how much disk space is free?
7. What happens when my disk fills up?
8. What happens if I use more disk space than I should?

Questions and answers

1.   How much disk space do we have?

   In late Fall 2005, a fileserver upgrade replaces the old 1TB (terabyte: 10¹², or a million million, bytes) filesystem with a new 10TB one, plus a real-time mirror to a duplicate 10TB filesystem in another building.

2.   Are disk quotas enforced?

   No, we feel strongly that disk quotas are simply foolish and arbitrary limits that mostly prevent people from getting legitimate work done.

3.   Do I have to pay for my disk space?

   No, not directly. Student computer fees paid as a tuition-fee surcharge each term supply a major part of the computer-operations budget, and research grants also contribute, so most users do indirectly pay for disk space.

4.   How much disk space can I use?

   Undergraduate accounts may each use a few tens of megabytes (1MB = 10⁶ = one million bytes). Graduate students, faculty, and staff may each use a few gigabytes (1GB = 10⁹ = one billion (thousand million) bytes).

   Users with projects that are expected to require many gigabytes should discuss their disk-space needs with the computer staff, since there are considerations of filesystem efficiency and filesystem backup media-size limits that may need to be addressed.

5.   How do I find out how much disk space I am using?

   The Unix du (disk usage) command answers that question:

   % cd                    # change to home directory
   % du -s -k              # ask for a summary (-s) in kilobytes (-k)
   15984   .               # I am using nearly 16MB
   % du -k | sort -k1nr    # ask for usage by directory, biggest first
   15984   .
   3635    ./thesis
   1967    ./.mozilla
   1056    ./.mozilla/firefox
   ...
   1       ./.Trash
   

6.   How do I find out how much disk space is free?

   The Unix df (disk free) command answers that question:

   % cd                    # change to home directory
   % df .                  # ask for free space in current (.) directory
       Filesystem           1k-blocks      Used Available Use% Mounted on
   suncore1:/home/0084      40487812  30661353   5777678  85% /a/suncore1/home/0084
   

   This disk has about 40GB of total space, of which only 5.7GB are free.

7.   What happens when my disk fills up?

   Disk resources are very active, and total usage fluctuates, sometimes dramatically over time intervals of a few minutes.

   The operating system is quite capable of surviving a disk-full situation, although it might send complaints to your terminal windows. Do not panic: graceful recovery is perfectly normal, although it may take some time to do so.

   If a disk-full situation prevents you from saving a file in a text editor, you can try to free some space in your own file tree by deleting unnecessary large files that you either no longer need, or can recreate automatically in a short time, or can recover from another system. Don't panic and delete files that your own fingers had to type!

   It may be possible to compress large files with bzip2 or gzip, but that may not work if there is insufficient space for both the original and the compressed files during the compression.

   If the disk-full situation persists for several minutes, then ask systems staff for help.

   If such help is not readily available, save your file(s) in the directory /var/tmp/, possibly first creating a subdirectory with your username there, and use the hostname command to find (and remember) the name of the computer that you are logged into, so that you can find your file(s) again. That directory is on that computer's local disk: it is not backed up, but it will survive reboots, and is not part of the filesystem that your login directory file tree is on.

   Do not save editor files in the /tmp directory: its contents are lost if the machine reboots. Even though our computer systems normally run for many months between reboots, Murphy's Law sometimes strikes at inopportune times.

8.   What happens if I use more disk space than I should?

   As long as your filesystem has enough free space, excess use is unlikely to be noticed. However, systems staff may monitor disk usage at any time, and may send users with large directory trees gentle, or perhaps not-so-gentle, reminders that some cleanup is in order.

   If disk-full problems persist, and the space needs are legitimate, then systems staff may compress large old files, or move selected large login directory trees to another filesystem with more space. Moves are normally done only after notification of the users involved: while the move is ultimately transparent, it does take some time to carry out, and data could be lost if files are still actively being updated during the move. However, in rare emergencies, file trees may be moved without notice.