Re: [PATCH 0/4] 3- and 4- copy RAID1

From: Hugo Mills <hugo@carfax.org.uk>
To: Duncan <1i5t5.duncan@cox.net>
Cc: linux-btrfs@vger.kernel.org
Subject: Re: [PATCH 0/4] 3- and 4- copy RAID1
Date: Wed, 18 Jul 2018 12:50:17 +0000	[thread overview]
Message-ID: <20180718125017.GC21293@carfax.org.uk> (raw)
In-Reply-To: <pan$47c54$5df59e3d$ab971dce$aa203b1@cox.net>

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On Wed, Jul 18, 2018 at 08:39:48AM +0000, Duncan wrote:
> Duncan posted on Wed, 18 Jul 2018 07:20:09 +0000 as excerpted:
> 
> >> As implemented in BTRFS, raid1 doesn't have striping.
> > 
> > The argument is that because there's only two copies, on multi-device
> > btrfs raid1 with 4+ devices of equal size so chunk allocations tend to
> > alternate device pairs, it's effectively striped at the macro level,
> > with the 1 GiB device-level chunks effectively being huge individual
> > device strips of 1 GiB.
> > 
> > At 1 GiB strip size it doesn't have the typical performance advantage of
> > striping, but conceptually, it's equivalent to raid10 with huge 1 GiB
> > strips/chunks.
> 
> I forgot this bit...
> 
> Similarly, multi-device single is regarded by some to be conceptually 
> equivalent to raid0 with really huge GiB strips/chunks.
> 
> (As you may note, "the argument is" and "regarded by some" are distancing 
> phrases.  I've seen the argument made on-list, but while I understand the 
> argument and agree with it to some extent, I'm still a bit uncomfortable 
> with it and don't normally make it myself, this thread being a noted 
> exception tho originally I simply repeated what someone else already said 
> in-thread, because I too agree it's stretching things a bit.  But it does 
> appear to be a useful conceptual equivalency for some, and I do see the 
> similarity.
> 
> Perhaps it's a case of coder's view (no code doing it that way, it's just 
> a coincidental oddity conditional on equal sizes), vs. sysadmin's view 
> (code or not, accidental or not, it's a reasonably accurate high-level 
> description of how it ends up working most of the time with equivalent 
> sized devices).)

   Well, it's an *accurate* observation. It's just not a particularly
*useful* one. :)

   Hugo.

-- 
Hugo Mills             | I gave up smoking, drinking and sex once. It was the
hugo@... carfax.org.uk | scariest 20 minutes of my life.
http://carfax.org.uk/  |
PGP: E2AB1DE4          |

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