Note: "permalinks" may not be as permanent as we would like,
direct links of old sources may well be a few messages off.
On Mon, Jan 23, 2012 at 01:08:57PM +0000, James Harper wrote: > I'm seeing something I didn't expect... I was getting very poor > results from the following command: > > time dd if=/dev/zero of=/dev/vg-drbd/test oflag=direct count=512 bs=512 > > in the order of 5 seconds. Then I added the following to my configuration: > > disk { > no-disk-barrier; > no-disk-flushes; > no-md-flushes; > } > > And the time dropped to under half a second, which is on par with > write performance when the secondary is offline. > > Where is the flush/barrier having an effect? I'm using protocol B so I > assumed that there would be no flush at all on the secondary in either > case as the packet is only supposed to have hit the secondary queue, > not necessarily the disk. I still want the data definitely on the disk > on the primary though so the flush should be happening there (I'm > prepared to lose data in the event of a power failure + simultaneous > destruction of the primary's disks). > > To summarise: > > primary&secondary online with all flushing/barriers enabled = bad performance > As above but secondary offline = good performance > Primary&secondary online with all flushing/barriers disabled = good performance > > My details are: > 2 x HP DL180's, each with 2 x 2TB 7200RPM SATA disks > Backing store is md (RAID0) on a partition > Protocol B > > I've tried all the other configuration tweaks I can think of, the only > think of is that flushing is having an effect on the secondary node??? > > Can anyone clarify the situation for me? See if that helps to understand what we are doing, and why: From: Lars Ellenberg Subject: Re: massive latency increases from the slave with barrier or flush enabled Date: 2011-07-03 08:50:53 GMT http://article.gmane.org/gmane.linux.network.drbd/22056 -- : Lars Ellenberg : LINBIT | Your Way to High Availability : DRBD/HA support and consulting http://www.linbit.com