[DRBD-user] blocking I/O with drbd

[Volker]

Hi Andreas,

>>                 no-disk-drain;
> 
> try replacing "no-disk-drain" by "no-md-flushes"

Thanks for your suggestion. Unfortunately setting that made it worse.
Shortly after

$ drbdadm adjust content

The load on the master went up to 4 and did not decrease afterwards.
After removing 'no-md-flushes' the load went down to around 1-1.5 again.

But:

There were two resyncs directly after activating and deactivating it. It
looked like below:

####
[root at nfs01 nfs]# cat /proc/drbd
version: 8.3.8 (api:88/proto:86-94)
GIT-hash: d78846e52224fd00562f7c225bcc25b2d422321d build by
mockbuild at builder10.centos.org, 2010-06-04 08:04:09
 0: cs:SyncTarget ro:Primary/Secondary ds:Inconsistent/UpToDate B r----
    ns:902760 nr:13965696 dw:14865000 dr:75668 al:1163261 bm:45844 lo:0
pe:0 ua:0 ap:0 ep:1 wo:d oos:1737728
	[================>...] sync'ed: 89.0% (1696/15332)M queue_delay: 0.0 ms
	finish: 0:00:52 speed: 32,928 (25,156) want: 25,600 K/sec
###

As you can see, the rate is at around 25MB, which is fine and fast
enough. The system-load on the master is not affected by this resync.

Why these resyncs happen and so much data is being resynced, is another
case. The nodes were disconnected for 3-4 Minutes which does not justify
so much data. Anyways...

One further note regarding the blocking-io:

After issueing the mentioned dd command

$ dd if=/dev/zero of=./test-data.dd bs=4096 count=10240
10240+0 records in
10240+0 records out
41943040 bytes (42 MB) copied, 0.11743 seconds, 357 MB/s

dd finishes within a couple of seconds (1-2) and the system-load does
not increase right away. It takes about 4-5 seconds for the load to
increase up to around 5-6. If i would issue a second dd-command right
after the first one finishes, the load would increase even higher than
5-6 with the second dd command being uninterruptible.

Interestingly dd _always_ reports speeds of 200-350MB which is obviously
not the case.

Any more ideas?

greetings
volker