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, Jun 24, 2008 at 12:27:30PM +0200, Andrei Neagoe wrote: > Hi, > > I was trying today to play with drbd's settings and benchmark the results in > order to obtain the best performance. > Here is my test setup: > 2 identical machines with sas storage boxes. Each machine has two 2TB device > (in my case /dev/sdb and /dev/sdc) that I mirror over drbd and on top of them > there's LVM set up. The nodes share a gbit link dedicated for drbd traffic. > After the initial sync which took something around 20 hours to finish, I've > created the LVM volume and formatted using ext3 FS. Then I started to play > around with params like al-extents, unplug-watermark, maxbuffers, max-epoch by > changing the values and doing a drbdadm adjust all on each node (of course > after copying the config file accordingly). In the begining it went pretty > well, maximum value attained by dd test over drbd was 28.9 MB/s: > > [root at erebus testing]# dd if=/dev/zero of=test.dat bs=1G count=1 oflag=dsync > 1+0 records in > 1+0 records out > 1073741824 bytes (1.1 GB) copied, 37.1114 seconds, 28.9 MB/s > > The configuration used is described in the end. After a couple more tests, I > noticed a big impact on performance, getting around 19-20 MB/s so I checked / > proc/drbd to see what's going on. Surprisingly, it was doing a full resync on > one of the disks. Problem is, I don't understand why, as normally it should > only resync discrepancies. if you change anything in the config file that changes "disk" parameters (like on-io-error, size, fencing, use-bmbv, ...), which causes drbdadm adjust to think it needs to detach/attach, and you do that while being primary, you get a full sync. this is unfortunate, and there should probably be a dialog to warn you about it. if you detach a Primary, then reattach, it will receive a full sync. you need to make it secondary first, if you want to avoid that. detaching, then reattaching a secondary will only receive an "incremental" resync, which typically is a few KB or nothing at all, depending on the timing. if this is not what happened for you, read the kernel log, typically drbd tells you why a resync was necessary. -- : Lars Ellenberg http://www.linbit.com : : DRBD/HA support and consulting sales at linbit.com : : LINBIT Information Technologies GmbH Tel +43-1-8178292-0 : : Vivenotgasse 48, A-1120 Vienna/Europe Fax +43-1-8178292-82 : __ please don't Cc me, but send to list -- I'm subscribed