On 2017-07-10 20:06, Mohamad Gebai wrote:
> On 07/10/2017 01:51 PM, Jason Dillaman wrote: On Mon, Jul 10, 2017 at 1:39
> PM, Maged Mokhtar <[email protected]> wrote: These are significant
> differences, to the point where it may not make sense
> to use rbd journaling / mirroring unless there is only 1 active client. I
> interpreted the results as the same RBD image was being concurrently
> used by two fio jobs -- which we strongly recommend against since it
> will result in the exclusive-lock ping-ponging back and forth between
> the two clients / jobs. Each fio RBD job should utilize its own
> backing image to avoid such a scenario.
That is correct. The single job runs are more representative of the
overhead of journaling only, and it is worth noting the (expected)
inefficiency of multiple clients for the same RBD image, as explained by
Jason.
Mohamad
Yes i expected a penalty but not as large. There are some use cases that
would benefit from concurrent access to the same block device, in vmware
ad hyper-v several hypervisors could share the same device which is
formatted via a clustered file system like MS CSV ( clustered shared
volumes ) or VMFS, which creates a volume/datastore that houses many
VMs.
I was wondering if such a setup could be supported in the future and
maybe there could be a way to minimize the overhead of the exclusive
lock..for example by having a distributed sequence number to the
different active client writers and have each writer maintain its own
journal, i doubt that the overhead will reach the values you showed.
Maged
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