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

[Max Reitz <mreitz@redhat.com>]

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