Re: how long should "btrfs device delete missing ..." take?

[Duncan <1i5t5.duncan@cox.net>]

Chris Murphy posted on Thu, 11 Sep 2014 15:25:51 -0600 as excerpted:

> On Sep 11, 2014, at 1:31 PM, Duncan <1i5t5.duncan@cox.net> wrote:
>> 
>> I wouldn't try defragging now, but it might be worthwhile to stop the
>> device delete (rebooting to do so since I don't think there's a cancel)
> 
> 'btrfs replace cancel' does exist, although I haven't tried it.

Btrfs replace cancel exists, yes, but does it work for btrfs device 
delete, which is what the OP used?

> Something isn't right though, because it's clearly neither reading nor
> writing at anywhere close to 1/2 the drive read throughput. I'm curious
> what 'iotop -d30 -o' shows (during the replace, before cancel), which
> should be pretty consistent by averaging 30 seconds worth of io. And
> then try 'iotop -d3 -o' and see if there are spikes. I'm willing to bet
> there's a lot of nothing going on, with occasional spikes, rather than a
> constant trickle.
> 
> And then the question is to find out what btrfs is thinking about while
> nothing is reading or writing. Even though it's not 5000+ snapshots, I
> wonder if the balance code (and hence btrfs replace) makes extensive use
> of fiemap that's causing this to go catatonic.
> http://comments.gmane.org/gmane.comp.file-systems.btrfs/35724

Not sure (some of that stuff's beyond me), but one thing we /do/ know is 
that btrfs has so far been focused mostly on features and debugging, not 
on optimization beyond the worst-cases, which themselves remain a big 
enough problem, tho it's slowly getting better.


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"Every nonfree program has a lord, a master --
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