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FAQ: Add question about running fsck on live filesystems.

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Richard W.M. Jones
2013-03-01 16:25:41 +00:00
parent 7d8b97559b
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examples/guestfs-faq.pod
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@@ -695,12 +695,6 @@ can't easily do with libguestfs.
=end html
=head2 Can I use C<guestfish --ro> as a way to backup my virtual machines?
Usually this is not a good idea. The question is answered in more
detail in this mailing list posting:
L<https://www.redhat.com/archives/libguestfs/2010-August/msg00024.html>
=head2 Why can I write to the disk, even though I added it read-only?
=head2 Why does C<--ro> appear to have no effect?
@@ -737,6 +731,37 @@ In guestfish, if you use the C<add> command, then disk is added
read-write (unless you specify the C<readonly:true> flag explicitly
with the command).
=head2 Can I use C<guestfish --ro> as a way to backup my virtual machines?
Usually this is I<not> a good idea. The question is answered in more
detail in this mailing list posting:
L<https://www.redhat.com/archives/libguestfs/2010-August/msg00024.html>
See also the next question.
=head2 Why can't I run fsck on a live filesystem using C<guestfish --ro>?
This command will usually I<not> work:
guestfish --ro -a /dev/vg/my_root_fs run : fsck /dev/sda
The reason for this is that qemu creates a snapshot over the original
filesystem, but it doesn't create a strict point-in-time snapshot.
Blocks of data on the underlying filesystem are read by qemu at
different times as the fsck operation progresses, with host writes in
between. The result is that fsck sees massive corruption (imaginary,
not real!) and fails.
What you have to do is to create a point-in-time snapshot. If it's a
logical volume, use an LVM2 snapshot. If the filesystem is located
inside something like a btrfs/ZFS file, use a btrfs/ZFS snapshot, and
then run the fsck on the snapshot. In practice you don't need to use
libguestfs for this -- just run C</sbin/fsck> directly.
Creating point-in-time snapshots of host devices and files is outside
the scope of libguestfs, although libguestfs can operate on them once
they are created.
=head2 What's the difference between guestfish and virt-rescue?
A lot of people are confused by the two superficially similar tools we