Is it me or is FreeBSD slower on Xen than Linux?

[Roger Pau Monné]

On Wed, Aug 17, 2016 at 10:19:05AM +0200, rainer at ultra-secure.de wrote:
> Am 2016-08-16 16:24, schrieb rainer at ultra-secure.de:
> > Am 2016-08-16 16:18, schrieb Roger Pau Monné:
> > 
> > > I'm not saying it's not possible, but are you sure this slowdown is
> > > caused
> > > by the disk? It's certainly a possibility, but I would like to make
> > > sure
> > > it's caused by that before jumping into conclusions.
> > > 
> > > Can you load the full database in RAM and perform the same test.
> > > TBH, I
> > > don't use MariaDB, so I'm not sure what's the best way to achieve
> > > this, but
> > > a quick search on google shows there are multiple ways. In any case,
> > > make
> > > sure with iostat that the database is not read from the disk.
> > 
> > I'll try to do some disk-benchmarks, when I can attach some bigger
> > disks.
> 
> on Ubuntu 14 with HVM:
> 
> dc3dd wipe=/dev/xvdc
> 
> dc3dd 7.1.614 started at 2016-08-17 09:38:17 +0200
> compiled options:
> command line: dc3dd wipe=/dev/xvdc
> device size: 104857600 sectors (probed)
> sector size: 512 bytes (probed)
> 53687091200 bytes (50 G) copied (100%), 464.642 s, 110 M/s
> 
> input results for pattern `00':
>    104857600 sectors in
> 
> output results for device `/dev/xvdc':
>    104857600 sectors out
> 
> dc3dd completed at 2016-08-17 09:46:01 +0200
> 
> 
> On FreeBSD 10.3-RELEASE-p6 with HVM:
> 
> Aug 17 09:57:52 bla-prod kernel: xbd4: 51200MB <Virtual Block Device> at
> device/vbd/51776 on xenbusb_front0
> Aug 17 09:57:52 bla-prod kernel: xbd4: features: write_barrier
> Aug 17 09:57:52 bla-prod kernel: xbd4: synchronize cache commands enabled.
> 
> dc3dd wipe=/dev/xbd4
> 
> and it's showing 8.something MB/s
> 
> Is this normal?

No, I don't think so, this is what I get using a slow USB 2.0 disk as the 
backend: (on Dom0 I get something between 70-80M/s, so there isn't much 
difference).

# dc3dd wipe=/dev/ada1

dc3dd 7.2.641 started at 2016-08-17 09:03:26 +0000
compiled options:
command line: dc3dd wipe=/dev/ada1
device size: 2097152 sectors (probed),    1,073,741,824 bytes
sector size: 512 bytes (probed)
  1073741824 bytes ( 1 G ) copied ( 100% ),   16 s, 65 M/s

input results for pattern `00':
   2097152 sectors in

output results for device `/dev/ada1':
   2097152 sectors out

dc3dd completed at 2016-08-17 09:03:42 +0000

> What may I be doing wrong?

TBH it's hard to tell, I don't know of any option that could cause this disk 
performance degradation. Do you also have ada* devices apart from the xbd* 
ones? I don't think it's going to make any difference, but could you try 
with the ada* block devices instead?

Roger.