Bug#650077: dpkg: The Installed-Size estimate can be wrong by a factor of 8 or a difference of 100MB
Package: dpkg
Version: 1.16.1.2
Severity: wishlist
Symptom
~~~~~~~
I just installed libjs-mathjax. According to its Installed-Size this
would just consume 16512KB. Now according to policy this is just an
estimate of course. But how accurate is it actually? So I installed said
package on ext3. Turns out /usr/share/javascript/mathjax takes up
127296KB and /usr/share/doc/mathjax takes another 1200KB. So our
estimate is wrong by a factor of 8 or a difference of 100MB. This
estimate is also used to determine whether the disk has enough space, so
if my disk just had 50MB left, aptitude would have tried to install this
package and failed.
The actual problem
~~~~~~~~~~~~~~~~~~
Problems with Installed-Size are not exactly new as discussion in
http://bugs.debian.org/534408 (unit for Installed-Size) and
http://bugs.debian.org/630533 (usage of du --apparent-size) have shown.
So what is different this time? Installing the very same package on a
btrfs yields a size that is much closer to the listed Installed-Size. (I
don't have any numbers on this.) So whatever dpkg puts into this field,
it *will* be wrong somewhere. The policy already mentions that this
estimate cannot be accurate everywhere, but in fact it will be wrong by
a factor of at least 2.5 (=sqrt(8)) or a difference of at least 50MB
(=100MB/2) somewhere. Any attempt to change the computation of this
value thus cannot fix this bug.
Discussion
~~~~~~~~~~
In the example of libjs-mathjax the reason for the huge difference is
the inclusion of a large number of very small files. Some filesystems
allocate a block for each of these files and others are able to store
multiple files in a block. A simple approach could be to include an
additional field ("Installed-Files"?) that returns the number of files
in the package. A second estimate for the Installed-Size would then be
given by the number of files times the block size. The maximum of both
estimates could be used. It would solve the immediate symptoms with
libjs-mathjax. It is not without problems though. For instance I
did not explain what block size to use. An administrator may have
different file systems set up for / and /usr. Also the question remains
whether this feature is worth the associated effort.
To get discussion going I pull in debian-policy@l.d.o.
Helmut
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