Re: [PLUG] zfs vs btrfs vs …

[Walt Mankowski <waltman@pobox.com>]

On Sat, Jan 31, 2015 at 12:16:02PM -0500, K.S. Bhaskar wrote:
> I am trying to create a virtual machine to be used to teach electronic
> health records for a class by giving each student a configured application
> s/he can then use, e.g., to record treatments.  The application includes a
> database of several GB.  I would like to give each student an individual
> database to work with, but would prefer not to have to give each student a
> separate copy of the database, when 99% of the database will be the same
> for all of them.
> 
> My current plan is to create the database on filesystem such as btrfs or
> zfs with a master copy of the database.  To add a student, a script would
> take a snapshot of the filesystem (or preferably just a sub-tree at
> directory in the file system) and mount the snapshot with copy-on-write in
> a different place for each student.  For example, if I had a /home/master,
> one might snapshot the master subdirectory and make it available at
> /home/adam and /home/eve.  Now Adam and Eve can each have a complete
> database, with one master copy, but each time one of them updates the
> database, the blocks in the filesystem on which the modified parts of the
> database live would be copied.  The additional space used is for
> modifications by each student.
> 
> I am trying to decide between zfs and btrfs.  One advantage of zfs over
> btrfs appears to be that the snapshots can be auto-mounted without editing
> /etc/fstab - with btrfs, adding 30 students to the class would seem to
> require 30 entries in /etc/fstab.  I have zero experience in zfs, and
> barely any experience with btrfs, so this will be a learning experience for
> me.
> 
> A couple of questions, please:
> 
>    - For this application, are there any considerations other than mounting
>    in choosing between zfs and btrfs?
>    - Should I explore some other alternative, along the lines of
>    unionfs-fuse?  A filesystem in userspace hs the advantage of not requiring
>    root to create a copy, but at least unionfs-fuse does copy-on-write at the
>    file level, not the block level, so each student would end up with a
>    complete copy of the database.  So, unionfs-fuse itself is out.
> 
> 
> Thank you very much, in advance, for advice, opinions, and pointers.

Other people have commented on zfs and btrfs, so let me start the
discussion on "...".  Given your description, I don't see the need to
try to optimize the disk space being used.  Wouldn't another option
just be to give everyone a complete copy of the database?  At several
GB per student, even a fairly big class could easily fit on an
inexpensive 1-2 TB HD.  Why not just do that instead of using
filesystems that are still poorly supported on Linux?

Walt

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