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, Feb 16, 2009 at 10:51:45AM +0000, Igor Neves wrote: > Hi, > > Some of you may know my problem since I have been discussing it with you > in the IRC in the last few days, but I will explain it. > > I'm doing some tests with drbd with xen virtual machines, our solution > was with vmware and drbd, but this solution it's with very low > performance compared to xen, and i'm trying to improve our solution to xen. > > I started my tests with xen on both machines (node1 & node2). Both > machines have 3 disks, sdb it's the drbd0. > > The disk can only deliver 102MB/sec writing, following seagate, and I > get about 105MB/sec from each disk in direct access. > I started my resouce in drbd and i'm able to sync at about 95MB/sec, > which it's very good. Testing the disk with dd or with your benchmark > tool called md, I get about 85MB/sec writing, which it's not bad. > > Now when I start the Xen Windows HVM Virtual Machine on node1 and I give > "/dev/drbd0" as a block device, it works great, but inside windows VM, I > get: > - With drbd rpm package 8.2.6 from redhat and with this configuration > "al-extents 1801" i get about 45MB/sec writing inside VM. > - With drbd rpm 8.2.7 and 8.3.0 from linbit and with this configuration > "al_extents 1801" i get about 4MB/sec writing inside VM. > - With source 8.2.7 and 8.3.0 from linbit and with "al_extents 1801" i > get again 4MB/sec writing inside VM. > > Now what I have found it's, all this tests were made with drbd in > connected & updated state. In the middle of the test if I do in node2 > "drbdadm disconnect resource" I get the state WForconnection and Update > on node1 and in that moment I start getting 85MB/sec. > > So inside Xen Windows HVM Virtual Machine in and with drbd under it as a > block device i get: > - drbd 8.2.6 from redhat and drbd connected = 45MB/sec, with drbd > disconnected = 85MB/sec > - drbd 8.2.7 from linbit and drbd connected = 4MB/sec, with drbd > disconnected = 85MB/sec > - drbd 8.3.0 from linbit and drbd connected = 4MB/sec, with drbd > disconnected = 85MB/sec in 8.2.7, we introduced a new method to ensure write ordering guarantees, which now needs to be explicitly disabled if you live on a "safe" device (battery backed write cache, or no cache at all), where this hurts performance. so where you had "no-disk-flushes" in drbd.conf before, you now need an additional "no-disk-barrier". if that does not help, or if you did not have "no-disk-flushes" before, let me know, and I try to come up with something else. > I know you can ask, it's not Xen block device driver? > - Yes it could, so i have added the other disk (exacly equal to the one > in drbd resource and with the same throughput) with direct access and > inside THE SAME windows virtual machine, I have 100MB/sec writing to the > disk, so the Xen block device it's going good and it's working like a > charm. Besides that Xen block device does not know "connected" and > "disconnected" as I explained in the early example. > > I have done this tests all again in node2 as Brian suggested, but i get > exactly the same values in the tests. > > So there is a problem here with drbd<->xen, something I have not managed > to find and could be a bug or maybe some bad configuration from my side. > > Please, someone with more experience help out on this. > > My kernel version it's "2.6.18-92.1.22.el5xen" and I'm with Centos 5 and > with all the latest updates available. > > Thanks, > Igor Neves -- : 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