LtspPersistentHome

Revision 10 as of 2006-11-06 19:57:19

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Summary

Rather than store their home directories up on a server, some users would like to plug in flash media, and run off their memory stick. This currently works on the live CD. We should look at getting this to work for LTSP thin clients as well.

Rationale

Edubuntu with LTSP is enjoying much success in schools, especially in third world countries where the cost of a full function PC plus software plus support may exceed a teacher's monthly or even yearly salary. Universities and colleges in these countries also struggle to fund adequate PC labs for the use of students, which impacts on teaching. Edubuntu with LTSP could make a great contribution in tertiary education, but we need to recognise that many students at colleges, and even some at schools, have some access to computers at home, in libraries, and elsewhere. If they can only access their home directories when they are on school or college premises then they are less able to exploit any other computer resources to do homework, projects, or research. As well, some systems administrators of public systems (say, for instance, libraries), may not want to have to manage home directories for users. This would provide the perfect oportunity for library patrons to have a persistent home directory.

Use cases

  • Joseph is studying journalism at a college. He gets an assignment to study and report on the employment opportunities that the growth of biofuel feedstock crops may offer to subsistence farmers. He starts developing his report on a memory stick inserted into a lab computer at college, then goes to the town library to access more information resources. He wants to further edit his report using a computer available within the library.

Scope

Affects:

  • ltsp-server-standalone
    • dependency on libpam_mount, libpam_script

Design

Currently, the persistent home system on the live cd requires the user to reformat/repartition their usb stick with an EXT3 filesystem. We'd like to avoid this, for support reasons. This requires us to create another layer of filesystem.

We'll need a filesystem shim that will provide standard unix filesystem semantics on top of a vfat filesystem. Things we'll need to handle that a fat formatted memory stick don't have are:

  • proper times (utime, mtime, ctime) witout constant updates to the memory stick (so as not to wear out the flash)
  • long filenames/directories simulation with 8.3 real names.
  • permissions, user, and group auth.
  • symlink and hardlink emulation.

Overall process will look something like the following:

  • The user will have to pre-prepare the stick for use.
    • This will be simple. No reformatting will be required, the user just has to create the /pershome directory.
    • Perhaps a simple shell script using zenity can be created to facilitate this.
  • User plugs in media into the workstation before login.
  • User logs in.
  • the pam module pam_script will run, and determine from either /etc/ltsp/persistent_homes, or $HOME/.persistent_home, that the user wishes to have a USB persistent home dir.
  • pam_mount executes a ltspfs mount to /media/username, which mounts the vfat formatted media.
  • pam_mount then executes a layered mount of a fuse based filesystem that behaves much like the rockridge extentions for CD's, in that it maps long filenames and permissions (but not userid or groupids) into the corresponding files in a /pershome directory on the vfat media. This directory then is mounted onto $HOME for the user.
    • If there is no /pershome directory, then we pop up a dialogue box indicating that the /pershome directory hasn't been created, possibly with instructions on how to do it.
  • On logout, reverse unmounting is done. The libpam-mount module does this. NOT VERIFIED

Implementation

  • Design format for /etc/ltsp/persistent_homes
    • simple text file of userids
    • $HOME/.persistent_home simply has to exist.
  • package pam_script, write persistent_home checking scripts, promote to main.
  • Write fuse-filesystem:
    • fuseexmp.c, simple loopback filesystem.
    • Need to handle getattr, readdir, & link (soft and hard) system calls.

    • rest are passed through.
    • prototype in Python with python-fuse bindings. Rewrite in C if performance is a problem.
  • pam_mount, fuse-python need to be promoted to main.

Code

  • fuse filesystem will need:
    • name munging routines
    • dictionary for mapping realname to fakename, file attributes
      • Save this out to the disk in a pickle.

Data preservation and migration

None.

Unresolved issues

BoF agenda and discussion

Need more input from the rest of the LTSP team.


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