[pve-devel] Default cache mode for VM hard drives

Stanislav German-Evtushenko ginermail at gmail.com
Thu May 28 09:22:12 CEST 2015

Hi Dietmar,

I did it couple of times already and everytime I had the same answer "upper
layer problem". Well, as we've done this long way up to this point I would
like to continue.

I have just done the same test with mdadm and not DRBD. And what I found
that this problem was reproducible on the software raid too, just as it was
claimed by Lars Ellenberg. It means that problem is not only related to
DRBD but to O_DIRECT mode generally when we don't use host cache and a
block device reads data directly from userspace.

The testcase is bellow.

1. Prepare

dd if=/dev/zero of=/tmp/mdadm1 bs=1M count=100
dd if=/dev/zero of=/tmp/mdadm2 bs=1M count=100
losetup /dev/loop1 /tmp/mdadm1
losetup /dev/loop2 /tmp/mdadm2
mdadm --create /dev/md0 --level=1 --raid-devices=2 /dev/loop{1,2}

2. Write data with O_DIRECT

./a.out /dev/md0

3. Check consistency with vbindiff

vbindiff /tmp/mdadm{1,2}      #press enter multiple times to skip metadata

And here we find that data on "physical devices" is different and md raid
did not catch this.

On Thu, May 28, 2015 at 7:40 AM, Dietmar Maurer <dietmar at proxmox.com> wrote:

> > What this means?
> I still think you should discuss that on the DRBD list.
Best regards,
Stanislav German-Evtushenko
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