On Thu, 2012-09-06 at 12:17 +0200, Harald Hoyer wrote: > Am 30.08.2012 00:58, schrieb Will Woods: > > /run will get mounted at $NEWROOT/run after switch_root, but it's not > > there yet. bind-mount it in place so updates for /run actually land in > > /run. > > > > Huh? Why do you want updates in /run ????? Short answer: because /tmp gets covered up by an empty tmpfs at (real) system startup, and we need to put them somewhere! Longer explanation: Historically, anaconda's runtime environment was read-only[1]. This makes sense, since it's usually squashfs (which doesn't do writes), and it's frequently running off a shiny, read-only plastic disc. But how do you fix bugs (or test bugfixes) without regenerating the image and burning a new disk? With updates images! To make updates work, anaconda sets its PYTHONPATH set to something like: /tmp/updates:$PYTHONPATH and sets GLADEPATH, PIXMAPPATH, LD_LIBRARY_PATH, etc. similarly. This way, updates images fetched with 'updates=http://...' get unpacked into /tmp/updates, and those files override the existing contents of the runtime image. Hooray! Bugfixes for read-only media! These days we *do* have the magic device-mapper overlay hack, so we can make a squashfs image read-write[2] and just overwrite files. But we still use the old update method for a couple reasons: 1) It already works, 2) it works with read-only filesystems (like squashfs), 3) overwriting existing files can be tricky[3]. ====== A SIDE DISCUSSION ABOUT UPDATES AND READONLY ROOT/USR ====== I admit this is all kind of a hack. It'd be much better if there was a generally supported systemwide place to put 'updates' - that is, things that override the existing system libraries/binaries/data/config in /, /etc, and /usr. Maybe a tmpfs mount at /usr/local would work, but that doesn't cover systemd configuration or ld.so. LD_LIBRARY_PATH could fix the latter, but it'd be better if we didn't have to change half a dozen $XXX_PATH variables to make this work. I seem to remember there was some work on readonly-root systems with the Stateless Linux stuff, but I'm not sure how/if that solves the problem here.. =================================================================== Anyway, to make the existing method keep working, we need to unpack the updates image to a tmpfs that stays around after switch-root. And that's what /run is for. So that's why we want to put the updates in /run. Does that answer the question? -w [1] Mostly because squashfs is read-only, but even if we weren't using squashfs the image might be on a CDROM or read-only NFS mount or something. [2] This actually requires that we put the runtime image into an ext4 image, and put *that* into squashfs - squashfs doesn't support writes at all, at the filesystem layer, and the device-mapper stuff redirects writes at the *block* layer.. [3] for example, some updates images contained /lib/libXXX.so. But then we overwrote the /lib -> /usr/lib symlink with the new '/lib' directory, thus breaking all the other libraries.. -- To unsubscribe from this list: send the line "unsubscribe initramfs" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
