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| -rw-r--r-- | examples/guestfs-faq.pod | 35 |
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diff --git a/examples/guestfs-faq.pod b/examples/guestfs-faq.pod index aeac482a..6c6db16b 100644 --- a/examples/guestfs-faq.pod +++ b/examples/guestfs-faq.pod @@ -583,6 +583,41 @@ 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? +Why does C<--ro> appear to have no effect? + +When you add a disk read-only, libguestfs places a writable overlay on +top of the underlying disk. Writes go into this overlay, and are +discarded when the handle is closed (or C<guestfish> etc. exits). + +There are two reasons for doing it this way: Firstly read-only disks +aren't possible in many cases (eg. IDE simply doesn't support them, so +you couldn't have an IDE-emulated read-only disk, although this is not +common in real libguestfs installations). + +Secondly and more importantly, even if read-only disks were possible, +you wouldn't want them. Mounting any filesystem that has a journal, +even C<mount -o ro>, causes writes to the filesystem because the +journal has to be replayed and metadata updated. If the disk was +truly read-only, you wouldn't be able to mount a dirty filesystem. + +To make it usable, we create the overlay as a place to temporarily +store these writes, and then we discard it afterwards. This ensures +that the underlying disk is always untouched. + +Note also that there is a regression test for this when building +libguestfs (in C<tests/qemu>). This is one reason why it's important +for packagers to run the test suite. + +=head2 Does C<--ro> make all disks read-only? + +I<No!> The C<--ro> option only affects disks added on the command +line, ie. using C<-a> and C<-d> options. + +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 What's the difference between guestfish and virt-rescue? A lot of people are confused by the two superficially similar tools we |