light: provisionary allow fetch during detach

docu/mars-manual.lyx

@@ -27547,6 +27547,219 @@ demand
  running.
 \end_layout
 
+\begin_layout Chapter
+Tips and Tricks
+\end_layout
+
+\begin_layout Section
+Creating Backups via Pseudo Snapshots
+\end_layout
+
+\begin_layout Standard
+When all your secondaries are all homogenously located in a standby datacenter,
+ they will be almost idle all the time.
+ This is a waste of computing resources.
+\end_layout
+
+\begin_layout Standard
+Since MARS Light is no substitute for a full-fledged backup system, and
+ since backups may put high system load onto your active side, you may want
+ to utilize your passive hardware resources in a better way.
+\end_layout
+
+\begin_layout Standard
+MARS Light supports this thanks to its ability to switch the 
+\family typewriter
+pause-replay
+\family default
+ 
+\emph on
+independently
+\emph default
+ from 
+\family typewriter
+pause-fetch
+\family default
+.
+\end_layout
+
+\begin_layout Standard
+The basic idea is simple: just use 
+\family typewriter
+pause-replay
+\family default
+ at your secondary site, but leave the replication of transaction logfiles
+ intact by deliberately 
+\emph on
+not
+\emph default
+ saying 
+\family typewriter
+pause-fetch
+\family default
+.
+ This way, your secondary replica (block device) will stay frozen for a
+ limited time, without loosing your redundancy: since the transaction logs
+ will continue to replicate in the meantime, you can start 
+\family typewriter
+resume-replay
+\family default
+ at any time, in particular when a primary-side incident should happen unexpecte
+dly.
+ The former secondary will just catch up by replaying the outstanding parts
+ of the transaction logs in order to become recent.
+\end_layout
+
+\begin_layout Standard
+However, some 
+\emph on
+details
+\emph default
+ have to be obeyed.
+ In particular, the current version of MARS Light needs an additional 
+\family typewriter
+detach
+\family default
+ operation, in order to release exclusive access to the underlying disk
+ 
+\family typewriter
+/dev/lv/$res
+\family default
+.
+ Future versions of MARS Light are planned to support this more directly,
+ without need for an intermediate 
+\family typewriter
+detach
+\family default
+ operation.
+\end_layout
+
+\begin_layout Standard
+\begin_inset Graphics
+	filename images/MatieresCorrosives.png
+	lyxscale 50
+	scale 17
+
+\end_inset
+
+Beware: 
+\family typewriter
+mount -o ro /dev/vg/$res
+\family default
+ can lead to 
+\series bold
+unnoticed write operations
+\series default
+ if you are not careful! Some journalling filesystems like 
+\family typewriter
+xfs
+\family default
+ or 
+\family typewriter
+ext4
+\family default
+ may replay their journals onto the disk, leading to 
+\emph on
+binary
+\emph default
+ differences and thus 
+\series bold
+destroying your consistency
+\series default
+ later when you re-enable 
+\family typewriter
+resume-replay
+\family default
+!
+\end_layout
+
+\begin_layout Standard
+\begin_inset Graphics
+	filename images/lightbulb_brightlit_benj_.png
+	lyxscale 12
+	scale 7
+
+\end_inset
+
+Therefore, you may use small LVM snapshots (only in such cases).
+ Typically, 
+\family typewriter
+xfs
+\family default
+ journal replay will require only a few megabytes.
+ Therefore you typically don't need much temporary space for this.
+ Here is a more detailed description of steps:
+\end_layout
+
+\begin_layout Enumerate
+
+\family typewriter
+marsadm pause-replay $res
+\end_layout
+
+\begin_layout Enumerate
+
+\family typewriter
+marsadm detach $res
+\end_layout
+
+\begin_layout Enumerate
+
+\family typewriter
+lvcreate --size 100m --snapshot --name ro-$res /dev/vg/$res
+\end_layout
+
+\begin_layout Enumerate
+
+\family typewriter
+mount -o ro /dev/vg/ro-$res /mnt/tmp
+\end_layout
+
+\begin_layout Enumerate
+Now draw your backup from 
+\family typewriter
+/mnt/tmp/
+\end_layout
+
+\begin_layout Enumerate
+
+\family typewriter
+umount /mnt/tmp
+\end_layout
+
+\begin_layout Enumerate
+
+\family typewriter
+lvremove -f /dev/vg/ro-$res
+\end_layout
+
+\begin_layout Enumerate
+
+\family typewriter
+marsadm up $res
+\end_layout
+
+\begin_layout Standard
+Hint: during the backup, the transaction logs will accumulate on 
+\family typewriter
+/mars/
+\family default
+.
+ In order to avoid overflow of 
+\family typewriter
+/mars/
+\family default
+ (c.f.
+ section 
+\begin_inset CommandInset ref
+LatexCommand ref
+reference "sec:Defending-Overflow"
+
+\end_inset
+
+), don't unnecessarily prolong the backup duration.
+\end_layout
+
 \begin_layout Chapter
 MARS for Developers
 \end_layout

kernel/sy_old/mars_light.c

@@ -1689,11 +1689,23 @@ int _update_file(struct mars_dent *parent, const char *switch_path, const char *
 		make_msg(msg_pair, "disabling fetch due to rmmod");
 		do_start = false;
 	}
+#if 0
+	/* Disabled for now. Re-enable this code after a new feature has been
+	 * implemented: when pause-replay is given, /dev/mars/mydata should
+	 * appear in _readonly_ form.
+	 * The idea is to _not_ disable the fetch during this!
+	 * You may draw a backup from the readonly device without losing your
+	 * redundancy, because the transactions logs will contiue to be updated.
+	 * Until the new feature is implemented, use
+	 * "marsadm pause-replay $res; marsadm detach $res; mount -o ro /dev/lv/$res"
+	 * as a workaround.
+	 */
 	if (do_start && !_check_allow(global, parent, "attach")) {
 		MARS_DBG("disabling fetch due to detach\n");
 		make_msg(msg_pair, "disabling fetch due to detach");
 		do_start = false;
 	}
+#endif
 	if (do_start && !_check_allow(global, parent, "connect")) {
 		MARS_DBG("disabling fetch due to disconnect\n");
 		make_msg(msg_pair, "disabling fetch due to disconnect");