[DRBD-user] Performance hit DRBD vs raw block device, even when disconnected

Wiebe Cazemier wiebe at halfgaar.net
Sat Jun 2 14:02:03 CEST 2012

Note: "permalinks" may not be as permanent as we would like,
direct links of old sources may well be a few messages off.


----- Original Message -----
> From: "Florian Haas" <florian at hastexo.com>
> To: "Wiebe Cazemier" <wiebe at halfgaar.net>
> Cc: drbd-user at lists.linbit.com
> Sent: Friday, 1 June, 2012 9:03:42 PM
> Subject: Re: [DRBD-user] Performance hit DRBD vs raw block device, even when disconnected
> 
> Sorry if I rant for a bit, but it's been _years_ since I wrote that
> part of the User's Guide, and it's been regurgitated a million times
> on this list, by myself and many others. Please, say it after me:
> 
> I will not use dd without oflag=direct or oflag=dsync.
> I will not use dd without oflag=direct or oflag=dsync.
> I will not use dd without oflag=direct or oflag=dsync.
> 
> You're kidding yourself. You're measuring *something*, a mix of
> caching effects and actual I/O, but not the throughput of your block
> device.
> 
> Please rerun your tests with oflag=direct, and then come back with
> updated results.
> 
> Florian

Sorry, I should have realized that...

Testing now yields more normal results:

1) about 25-30 MB/s on both nodes.
2) Higher speeds (about 50 MB/s) when DRBD device is in disconnect mode.

If I invalidate my test device (causing a resync), it syncs at about 60 MB/s, the speed I limited it to. I don't understand why normal throughput is so much lower. Fiddling with max-buffers, max-epoch-size and unplug-watermark doesn't really seem to influence the 25 MB/s much.

To give more information about the hosts:

Host1: Supermicro (mainboard: X8SIL-F). 3Ware 9650 SE SATA2 hardware RAID6, 5x 2TB disks. Xeon(R) CPU X3430, 2.40GHz. Machine is Xen dom0 and dom0 has 1 core.

Host2: Dell Poweredge R200. 2x250GB Linux MD RAID1. Xeon CPU X3220, 2.40GHz. Machine is Xen dom0 and dom0 has 4 cores.

Both nodes reach higher network and disk speeds then I'm getting here.

Regards,

Wiebe



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