On Mar 28, 2020, at 12:26 PM, Nir Soffer <nsoffer@redhat.com> wrote:

On Sat, Mar 28, 2020 at 1:59 PM Strahil Nikolov <hunter86_bg@yahoo.com> wrote:

On March 28, 2020 11:03:54 AM GMT+02:00, Gianluca Cecchi <gianluca.cecchi@gmail.com> wrote:

On Sat, Mar 28, 2020 at 8:39 AM Strahil Nikolov <hunter86_bg@yahoo.com>
wrote:

On March 28, 2020 3:21:45 AM GMT+02:00, Gianluca Cecchi <
gianluca.cecchi@gmail.com> wrote:

[snip]

Actually it only happened with empty disk (thin provisioned) and

sudden

high I/O during the initial phase of install of the OS; it didn't
happened
then during normal operaton (even with 600MB/s of throughput).

[snip]

Hi Gianluca,

Is it happening to machines with preallocated disks or on machines

with

thin disks ?

Best Regards,
Strahil Nikolov

thin provisioned. But as I have tro create many VMs with 120Gb of disk
size
of which probably only a part during time will be allocated, it would
be
unfeasible to make them all preallocated. I learned that thin is not
good
for block based storage domains and heavy I/O, but I would hope that it
is
not the same with file based storage domains...
Thanks,
Gianluca

This is normal - gluster cannot allocate fast enough the needed shards (due to high IO),  so the qemu pauses  the VM until  storage  is available  again .

I don't know glusterfs internals, but I think this is very unlikely.

For block storage thin provisioning in vdsm, vdsm is responsible for allocating
more space, but vdsm is not in the datapath, it is monitoring the allocation and
allocate more data when free space reaches a limit. It has no way to block I/O
before more space is available. Gluster is in the datapath and can
block I/O until
it can process it.

Can you explain what is the source for this theory?

You can think about VDO (with deduplication ) as a  PV for the  Thin LVM and this way you can preallocate your VMs , while saving space (deduplication, zero-block elimination  and even compression).
Of  course, VDO will reduce  performance (unless  you have battery-backed write cache and compression is disabled),  but  tbe benefits will be alot more.

Another approach is to increase the shard size - so gluster will create fewer  shards,  but allocation on disk will be higher.

Best Regards,
Strahil Nikolov
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