Note: "permalinks" may not be as permanent as we would like,
direct links of old sources may well be a few messages off.
On Tue, Nov 11, 2008 at 12:35:45PM +0100, Arnold Hendriks wrote: > Arnold Hendriks wrote: >> I'm crudely testing whether replication works at all using 'dd': >> at m1: dd if=/dev/urandom of=/dev/drbd4 bs=512 count=1 conv=fsync >> oflag=sync seek=0 >> at m2: less -f /dev/drbd4 >> >> Seeing the bytes change, I assume things are working. Now I start >> iscsitarget (ietd daemon) which has been configured to export >> /dev/drbd4, and without doing anything else on the machines, it seems >> the syncs between m1 and m2 have stopped. Doing a new 'dd' on m1 >> doesn't update the data I see on /dev/drbd4 on m2 anymore. After I >> stop iscsi-target, I can see the changes I made on m2. Starting >> iscsi-target makes the problem reappear. > I discovered something new, after Jürgen's hint about trying to overflow > the caches. Running a 8GB dd showed all drives writing in iostat, so > there was definately data coming in. But still I didn't see changes > > However, after running 'echo 1 > /proc/sys/vm/drop_caches' to free the > pagecache (which I found on http://linux-mm.org/Drop_Caches) the changes > _are_ visible with 'less -f /dev/drbd4'. So with this test sequence on > my system: > > m2: less -f /dev/drbd4 (I see data starting with <96>) > m1: dd if=/dev/urandom of=/dev/drbd4 bs=512 count=1 conv=fsync > oflag=sync seek=0 > m2: less -f /dev/drbd4 (I still see data starting with <96>) > m2: echo 1 > /proc/sys/vm/drop_caches > m2 : less -f /dev/drbd4 (I see new data) > > So somehow it seems drbd did manage to put the data on the underlying > block device (or it got there by some other route unknown to me), but is > not showing it on /dev/drbd4 until I drop the caches. But it seems the > problem has to be on 'm2' somewhere.. > > Any pointers? are you sure you did not use "dd ... oflag=direct" ? if you did, you bypassed the cache, and your results would be expected. -- : Lars Ellenberg : LINBIT | Your Way to High Availability : DRBD/HA support and consulting http://www.linbit.com DRBD® and LINBIT® are registered trademarks of LINBIT, Austria. __ please don't Cc me, but send to list -- I'm subscribed