add a note to README.checkarray about I/O scheduling

Signed-off-by: martin f. krafft <madduck@debian.org>

1 files changed, 13 insertions, 1 deletions

diff --git a/debian/README.checkarray b/debian/README.checkarray
index 8cefe016..8071a4d6 100644
--- a/debian/README.checkarray
+++ b/debian/README.checkarray
@@ -7,6 +7,18 @@ in the morning. This is realised by asking cron to wake up every Sunday with
 /etc/cron.d/mdadm, but then only running the script when the day of the month
 is less than or equal to 7. See #380425.
 
+Cron will try to run the check at "idle I/O priority" (see ionice(1)), so that
+the check does not overload the system too much. Note that this will only
+work if all the component devices of the array employ the (default) "cfq" I/O
+scheduler. See the kernel documentation[0] for information on how to verify
+and modify the scheduler. checkarray does not verify this for you.
+
+  0. http://www.kernel.org/doc/Documentation/block/switching-sched.txt
+
+If you manually invoke checkarray, it runs with default I/O priority. Should
+you need to run a check at a higher (or lower) I/O priority, then have a look
+at the --idle, --slow, --fast, and --realtime options.
+
 'check' is a read-only operation, even though the kernel logs may suggest
 otherwise (e.g. /proc/mdstat and several kernel messages will mention
 "resync"). Please also see question 21 of the FAQ.
@@ -18,4 +30,4 @@ However in the absence of read errors it is read-only.
 
 You can cancel a running array check with the -x option to checkarray.
 
- -- martin f. krafft <madduck@debian.org>  Mon, 08 Jan 2007 02:07:19 +0100
+ -- martin f. krafft <madduck@debian.org>  Thu, 02 Sep 2010 10:27:29 +0200