doc: Modified a note section in rbd-snapshot doc.

Elaborates the note section a bit more as to why stopping I/O before
taking a snapshot is recommended and how to do it.

Signed-off-by: Nilamdyuti Goswami <ngoswami@redhat.com>

diff --git a/doc/rbd/rbd-snapshot.rst b/doc/rbd/rbd-snapshot.rst
index 96169efa51969f4374f6fb53f7790c763522d32a..2e5af9f181f7efe83dd4f86d2b14731a2db4ba78 100644 (file)
--- a/doc/rbd/rbd-snapshot.rst
+++ b/doc/rbd/rbd-snapshot.rst
@@ -14,9 +14,14 @@ command and many higher level interfaces, including `QEMU`_, `libvirt`_,
 
 .. important:: To use use RBD snapshots, you must have a running Ceph cluster.
 
-.. note:: **STOP I/O BEFORE** snapshotting an image.
-   If the image contains a filesystem, the filesystem must be in a
-   consistent state **BEFORE** snapshotting.
+.. note:: If a snapshot is taken while `I/O` is still in progress in a image, the
+   snapshot might not get the exact or latest data of the image and the snapshot
+   may have to be cloned to a new image to be mountable. So, we recommend to stop
+   `I/O` before taking a snapshot of an image. If the image contains a filesystem,
+   the filesystem must be in a consistent state before taking a snapshot. To stop
+   `I/O` you can use `fsfreeze` command. See `fsfreeze(8)` man page for more details.
+   For virtual machines, `qemu-guest-agent` can be used to automatically freeze
+   filesystems when creating a snapshot.
    
 .. ditaa:: +------------+         +-------------+
            | {s}        |         | {s} c999    |