Re: [Patch 0/5] I/O statistics through request queues

[Martin Peschke <mp3@de.ibm.com>]

Jens Axboe wrote:
>>> Thanks! Also note that you do not need to log every event, just register
>>> a mask of interesting ones to decrease the output logging rate. We could
>>> so with some better setup for that though, but at least you should be
>>> able to filter out some unwanted events.
>> ...and consequently try to scale down relay buffers, reducing the risk of
>> memory constraints caused by blktrace activation.
> 
> Pretty pointless, unless you are tracing lots of disks. 4x128kb gone
> wont be a showstopper for anyone.

per (online) CPU and device?

>>>> However, a fast network connection plus a second system for blktrace
>>>> data processing are serious requirements. Think of servers secured
>>>> by firewalls. Reading some counters in debugfs, sysfs or whatever
>>>> might be more appropriate for some one who has noticed an unexpected
>>>> I/O slowdown and needs directions for further investigation.
>>> It's hard to make something that will suit everybody. Maintaining some
>>> counters in sysfs is of course less expensive when your POV is cpu
>>> cycles.
>> Counters are also cheaper with regard to memory consumption. Counters
>> are probably cause less side effects, but are less flexible than
>> full-blown traces.
> 
> And the counters are special cases and extremely inflexible.

Well, I disagree with "extremely".
These statistics have attributes that allow users to adjust data
aggregation, e.g. to retain more detail in a histogram by adding
buckets.