[GSoC][PATCH v2 3/6] qemu_domainjob: `maxQueuedJobs` added to `qemuDomainJobPrivate`

[Michal Privoznik]

On 8/18/20 10:48 AM, Erik Skultety wrote:
> On Mon, Aug 17, 2020 at 10:37:18AM +0530, Prathamesh Chavan wrote:
>> Reference to `maxQueuedJobs` required us to access
>> config of the qemu-driver. And creating its copy in
>> the `qemuDomainJob` helped us access the variable
>> without referencing the driver's config.
>>
>> Signed-off-by: Prathamesh Chavan <pc44800 at gmail.com>
>> ---
>>   src/qemu/qemu_domain.c    |  5 ++++-
>>   src/qemu/qemu_domainjob.c | 13 +++++++------
>>   src/qemu/qemu_domainjob.h |  4 +++-
>>   3 files changed, 14 insertions(+), 8 deletions(-)
>>


> Looking at this again, I don't think we want to move any of the jobs_queued or
> maxQueuedJobs attributes to the job structure, I'm sorry I didn't spot this
> right away in the previous revision of the series, I take responsibility for
> it.
> The job structure represents a single job, so from the design POV,
> referencing both jobs_queued (which is really tied to a domain) and
> maxQueuedJobs (which is a global driver setting) from within the job structure
> simply doesn't feel right. Instead, I think qemuDomainObjBeginJobInternal will
> simply have to be split in parts specific to QEMU that have access to the
> driver and hypervisor-agnostic bits that can be moved to src/hypervisor.

Actually, I think maxQueuedJobs is job specific and not a global driver 
setting. I mean, I can imagine other drivers benefiting from it too.
It's true that we currently don't allow specifying different values for 
different domains and only apply the setting from qemu.conf, but in the 
future we might make this available in domain XML somehow [1] and thus 
be per domain. Moreover, I can imagine users wanting to set the limit 
for volumes too [2].

Michal

1: this refers to discussion we had on the list (too lazy to find the 
link, sorry) about the qemu:///embed. So far, if an user wants to set 
some values, they have to create qemu.conf in the root before connecting 
to the embed driver (so that the driver loads them). The idea was to 
allow them to specify everything in domain XML so no file needs to be 
created.

2: although, it's not as bad as it used to be, but with rotational disks 
having two processes accessing different portions of a file resulted in 
very poor performance as the head had to jump up & down.