Re: [PATCH 0/7] iotests/129: Fix it

[Max Reitz]

On 13.01.21 15:31, Vladimir Sementsov-Ogievskiy wrote:
> 13.01.2021 17:06, Max Reitz wrote:
> > Hi,
> >
> > There are some problems with iotests 129 (perhaps more than these, but
> > these are the ones I know of):
> >
> > 1. It checks @busy to see whether a block job is still running; however,
> >     block jobs tend to unset @busy all the time (when they yield).
> >     [Fixed by patch 3]
> >
> > 2. It uses blockdev throttling, which quite some time ago has been moved
> >     to the BB level; since then, such throttling will no longer affect
> >     block jobs.  We can get throttling to work by using a throttle filter
> >     node.
> >     [Fixed by patch 4]
> >
> > 3. The mirror job has a large buffer size by default.  A simple drain
> >     may lead to it making significant process, which is kind of
> >     dangerous, because we don’t want the job to complete.
>
> Not quite clear to me. iotest 129 wants to mirror 128M of data. Mirror by
> default will have 1M chunk size and maximum of 16 parallel requests. So 
> with
> throttling (even if throttling can't correctly handle parallel requests)
> we will not exceed 16M of progress.. Why we need limiting buffer size?

It does exceed 16M of progress; without the limit, I generally see 
something between 16M and 32M.

Now, that still is below 128M, but it’s kind of in the same magnitude. 
I don’t feel comfortable with that, especially given it’s so easy to 
limit it to much less (buf_size=64k makes the job proceed to 128k).

Also, maybe the default is increased in the future.  Increasing the 
chunk size by 4 would mean that it might be possible to reach 128M.

I find not relying on the default better.

Max