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

Stanislav German-Evtushenko ginermail at gmail.com
Thu May 28 09:31:14 CEST 2015

Moreover, if you create ext3 on top of md0 and repeat then raid array
becomes inconsistent too.

# Additional steps:
mkfs.ext3 /dev/md0
mkdir /tmp/ext3
mount /dev/md0 /tmp/ext3
./a.out /tmp/ext3/testfile1

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

On Thu, May 28, 2015 at 10:22 AM, Stanislav German-Evtushenko <
ginermail at gmail.com> wrote:

> 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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