Note: "permalinks" may not be as permanent as we would like,
direct links of old sources may well be a few messages off.
Hi,
There seems to be a number of people experiencing this problem (myself included). I've only seen one response offering any sort of suggestions so far (http://article.gmane.org/gmane.comp.linux.drbd/22060), but I tried changing my configuration to more closely match what Ulrich mentioned in that message and saw no change in performance.
It would be great if a developer (or someone with a similar level of expertise/authority) could chime in on this, but I haven't seen that happen yet.
Please do post to the list if you find a solution!
Thanks,
Zev
On Jul 8, 2011, at 8:00 AM, Mark Dokter wrote:
> Hello!
>
> I have a rather complex setup and experience severe performance
> degradation, especially on small writes.
>
> ------------------------------------
> The scenario:
> - Two xen servers with:
> - kernel 2.6.32 (xen-stable git)
> - xen 4.1.0
> - ubuntu 11.04
> - backported drbd 8.3.7
> - domUs using pv-on-hvm drivers
>
> - DRBD volumes for the domUs:
> HW RAID6 -> LVM -> DRBD
>
> - Sync Network:
> - dedicated 1Gbit/s ethernet (switched)
>
> - DRBD volume config:
>
> resource pegasus {
> device /dev/drbd8;
> disk /dev/data/pegasus;
> meta-disk /dev/data/drbd-metadata[8];
>
> net {
> allow-two-primaries;
> max-buffers 8000;
> max-epoch-size 8000;
> sndbuf-size 0;
> unplug-watermark 16;
> }
>
> syncer {
> al-extents 3389;
> rate 110M;
> }
>
> on xen01 {
> address 172.16.128.1:7797;
> }
>
> on xen02 {
> address 172.16.128.2:7797;
> }
> }
>
> ------------------------------------
>
> I have several domUs set up like this. The worst slowdowns are with
> - mailserver (postfix/cyrus)
> - mysql
> - copying a lot of small files
>
> A small test I ran(disk/net i/o checked with dstat):
> - copied the mailman directory (110 lists, 6.5 GB of data)
> cp -a mailman mailman-backup
> - after about 15 minutes I had a look how far the copying got:
> about 1 GB copied.
> - in the dom0 there was about 1 MB/s traffic on the sync network.
> Same speeds as the disk write performance in the domU
> - I disconnected the DRBD volume on the second xen dom0
> (the one the virtual machine was not running on)
> drbdadm down pegasus
> - the disk write performance in the domU went up to around 30 - 40 MB/s
>
> The 30-40 MB/s aren't really fast either, but that'd be bearable. When I
> reconnected the volume after the copying was done, the resync went quite
> fast (also with those 30 - 40 MB/s).
> I know, the numbers aren't that exact, but it's enough to see the big
> difference when the DRBD volumes are in connected/disconnected state.
>
> I already did quite some research on that issue (that's how I came up
> with that DRBD config), but didn't succeed in fixing it. Maybe someone
> on this list has suggestions?
>
> Thanks,
> Mark
>
>
>
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