[DRBD-user] write performance issues

Zev Weiss zweiss at scout.wisc.edu
Sun Jul 10 22:26:51 CEST 2011

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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> 




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