Am 06.09.2012 19:26, schrieb Will Woods: > 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.. > Ok. Question answered. Will apply. -- 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
