Re: [Xen-users] Alternatives to a dual-primary DRBD setup

[Daniel Meszaros <meszi@xxxxxxxxxxx>]

Hi!

Am 05/21/2011 02:16 AM, schrieb James Harper:
> I would consider splitting up the storage and VM functions onto separate
> servers. So 2 servers running DRBD and exporting LUN's via iSCSI (or AoE
> or FCoE or whatever works for you) and then 2 (or more) servers running
> VMs. This obviously needs more hardware though.
Unfortunately this is not possible at the moment due to limited budget. 
Besides this I expect other performance issues when introducing another 
layer like iSCSI or AoE to the game.
What I could do is to take for instance two harddisks into an own RAID1 
being DRBD-replicated parallelly to the data but I guess this would make 
no difference to my current situation in the end.
I also thought about leaving the DRBD idea behind and to setup two 
independent servers, one holding the system backups and one being the 
productive machine if my systems couldn't cope with the DRBD thing. :-/
Another idea is to put the virtual systems onto the harddrive that the 
Xenserver host resides on (and not replicating them on-the-fly) and just 
leaving data on the DRBD-synced RAID5s.
The last both ideas would make XenMotion impossible which was the reason 
why I decided to run the Citrix solution. In that case I could possibly 
also run some newer (= better?) Xen version, i.e. under Debian Squeeze. :-/
Or some other idea that hasn't yet been considered...


> Having said that, can you elaborate on the "sync issues" you are having
> with DRBD? Anything that breaks under high load is still IMHO a bug and
> can probably also break under low load, just less often, Are you running
> the latest version?
No, it isn't the latest DRBD version (= 8.3.8.1). Also my Citrix 
Xenserver isn't latest. It is still 5.6 without any FP or SP.
When I set up the virtualization system I saw everything running fine 
but the host machines weren't filled with virtual guests yet, 
unfortunately. It must be the (raised) amount of machines having reached 
an I/O performance consumption level that somewhen started concurring 
with the DRBD sync.
At least no system or data got harmed yet ... *knock knock* ... but it's 
not redundant at the moment. :-(
Currently when running a benchmark from one dom0 to the other (with 
"netio") I get throughput rates of > 200 MByte/s. And this is the speed 
that I used to have with DRBD, too. But as soon as I start syncing from 
the "younger" primary to the other DRBD machine my virtual guests' logs 
show timeout error messages.
In my investigations I read that a 10 GbE NIC can cause high CPU loads 
when not being configured properly. Is there possibly a way to reserve 
more of the CPU time and/or RAM for the Xen host/ dom0? I have plenty of 
CPU cores in the hosts and RAM is also far from being exceeded, still.
CU,
Daniel.

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