As described in QubesOS/qubes-issues#3758, a VM may fail to start
if the volume is large. This because the whole volume is read to
ensure it's empty (=all zeros).
This changes limits the check to the first 1 GiB of the private
volume. As additional safety, a scan with blkid is done.
Fixes https://github.com/QubesOS/qubes-issues/issues/3758
On first VM's boot, setup-rwdev.sh script create filesystem on
/dev/xvdb. But it does so only after checking if /dev/xvdb is really
empty, by comparing it to /dev/zero. Speed up reads from /dev/zero bu
using larger blocks (default of head - 8k, instead of explicit 512).
This speed up the check over 5 times.
When trying offline resize2fs, it require running fsck first, which
takes time, especially on large volumes. And in most cases, resize2fs
will notice that no action is needed - after wasting some time on fsck.
To remedy this, use resize2fs in online mode (on mounted filesystem).
And drop fsck call if it fails (filesystem is already mounted
read-write, running fsck isn't good idea).
But do not remove fsck call completely - still call it, but without '-f'
flag, so it run actual check only when really needed (unclean shutdown,
last check far in the past etc).
FixesQubesOS/qubes-issues#979FixesQubesOS/qubes-issues#2583
