[Gluster-devel] Proposal for change regarding latency calculation

[Vipul Nayyar]

Hello,

Following is a proposal for modifying the io profiling capability of the io-stats xlator. I recently sent in a patch(review.gluster.org/#/c/8244/) regarding that, which uses the already written latency related functions in io-stats to dump info through meta and added some more data containers which would track some more fops related info each time a request goes through io-stats. Currently, before the io-stats' custom latency functions can run, the measure_latency and count_fop_hits option should be enabled. I propose to remove these two options entirely from io-stats.

In order to track io performance, these options should be enabled all the time, or removed entirely, so that a record of io requests can be kept since mount time, else enabling these options only when it is required will not give you the average statistics over the whole period since the start. This is based on the methodology of Linux kernel itself, since it internally maintains the io statistics data structures all the time and presents it via /proc filesystem whenever required. Enabling of any option is not required, and the data available represents statistics since the boot time.

I would like to know the views over this, if having io-stats profiling info available all the time would be a good thing?

Apart from this, I was going over latency.c in libglusterfs, which does a fine job of maintaining latency info for every xlator and encountered an anomaly which I thought should be dealt with. The function gf_proc_dump_latency_info which dumps the latency array for the specified xlator consists of a last line which in the end flushes this array through memset after every dump. That means, you get different latency info every time you read the profile file in meta. I think, flushing the data structure after every dump is wrong since, you don't get overall stats since one enabled the option at the top of meta, and more importantly, multiple applications reading this file can get wrong info, since it gets cleared after one read only.

If my reasons seem apt for you, I'll send a patch over for evaluation.

Regards

Vipul Nayyar 
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