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Re: [Qemu-block] [PATCH 2/2] qapi: add block latency histogram interface

From: Eric Blake
Subject: Re: [Qemu-block] [PATCH 2/2] qapi: add block latency histogram interface
Date: Tue, 6 Feb 2018 13:02:37 -0600
User-agent: Mozilla/5.0 (X11; Linux x86_64; rv:52.0) Gecko/20100101 Thunderbird/52.5.2

On 02/06/2018 12:06 PM, Vladimir Sementsov-Ogievskiy wrote:
06.02.2018 18:50, Eric Blake wrote:
On 02/06/2018 08:07 AM, Vladimir Sementsov-Ogievskiy wrote:
Set (and clear) histogram through new command
block-latency-histogram-set and show new statistics in
query-blockstats results.

The commit message mentioned that you can set and clear histogram tracking; how does this interface let you clear things?  By passing a 0-length latency array?  If you violate the constraint (pass non-ascending points, for example), does the previous latency map get wiped out or is it still preserved unchanged?

On error nothing is changed.

By "clear" I mean zeroing statistics, not removing the whole histogram. And to zero statistics you can call set with the same latency array. There is no way to remove histogram at all.. We can add block-latency-histogram-unset later if needed.

Maybe "set (or restart) histogram collection points" might read better? I also don't think we need a new command; if you make 'latency' optional, then omitting it can serve to stop collecting statistics altogether, without needing a new command (then again, if you do that, now the command is used to "set, restart, and clear histogram collection").

Eric Blake, Principal Software Engineer
Red Hat, Inc.           +1-919-301-3266
Virtualization:  qemu.org | libvirt.org

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