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James Antill - Post a comment

Sep. 2nd, 2008

10:06 pm - A quick explanation of package sizes in yum and rpm

It's pretty common to think that a specific thing always has a specific size, and people tend to think of an "rpm package" as a single object thus. the it's common to ask what is "the size of an rpm". However if you have a 1MB text file, and gzip compresses it to 50KB which you then upload to a HTTP server you now have at least 3 different sizes: text size; compressed size and upload size (includes HTTP headers etc.) and asking for the size. So it is with rpm packages, and their many sizes.

The three common sizes of an rpm package

I'm going to use the yum package object notation to explain the common different sizes, as those are the easiest to see/use (see my previous post on how to simply get package objects from yum):

  • An .rpm file package has pkg.archivesize (rpm TAG RPMTAG_ARCHIVESIZE), pkg.installedsize (rpm TAG RPMTAG_SIZE) and (for yum available package objects) pkg.packagesize (a stat of the .rpm file).

  • Installed rpm packages have pkg.archivesize (rpm TAG RPMTAG_ARCHIVESIZE) and pkg.packagesize (rpm TAG RPMTAG_SIZE). pkg.installedsize doesn't exist, because an installed package can't be installed.

How are those values calculated?

rpm calculates the RPMTAG_SIZE value as the simple summation of the sizes of all the files within the rpm. rpm calculates the RPMTAG_ARCHIVESIZE value as the value of the cpio archive within the rpm, after decompression. These values are often very close. As I said above the pkg.packagesize value for .rpm files is just that value returned by stat(2).

So, what is pkg.size and why does it change?

Within yum there is a pkg.size, which maps to pkg.packagesize, which is used in all UI code that just wants to know "the size" of a package. This value has the property that the value you get for "need to download X bytes to install" and "will free up X bytes on removal" is correct, which is what most people want to see most of the time. However using this value does mean that the "the size of the rpm package" changes after you install an rpm, so it can be confusing if you try and compare pkg.size values between installed and available packages (either via. yum info, or looking at the values presented when installing a new kernel and removing an old one, say).

So, what's the best way to compare the size of an rpm package?

As you can see above, archivesize is the same for an available package and an installed one. So if you install the yum-lsit-data plugin, you can use "yum --showduplicates info-archive-sizes <pkgs>". Also you can always compare with just the available packages (and not installed ones), for instance "repoquery --show-duplicates --qf '%{nevra} %{archivesize} %{size}\n' <pkgs>"

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