Re: raid10 redundancy

From: antlists <antlists@youngman.org.uk>
To: Phillip Susi <phill@thesusis.net>
Cc: Roy Sigurd Karlsbakk <roy@karlsbakk.net>,
	Linux Raid <linux-raid@vger.kernel.org>
Subject: Re: raid10 redundancy
Date: Wed, 19 May 2021 00:48:26 +0100	[thread overview]
Message-ID: <d7c8b22d-f74a-409e-4e08-46240bb815e4@youngman.org.uk> (raw)
In-Reply-To: <874kf0yq31.fsf@vps.thesusis.net>

On 18/05/2021 17:05, Phillip Susi wrote:
> 
> Wols Lists writes:
> 
>> When rebuilding a mirror (of any sort), one block written requires ONE
>> block read. When rebuilding a parity array, one block written requires
>> one STRIPE read.
> 
> Again, we're in agreement here.  What you keep ignoring is the fact that
> both of these take the same amount of time, provided that you are IO bound.
> 
And if you've got spinning rust, that's unlikely to be true. I can't 
speak for SATA, but on PATA I've personally experienced the exact 
opposite. Doubling the load on the interface absolutely DEMOLISHED 
throughput, turning what should have been a five-minute job into a 
several-hours job.

And if you've got many drives in your stripe, who's to say that won't 
overwhelm the i/o bandwidth. Your reads could be 50% or less of full 
speed, because there isn't the back end capacity to pass them on.

Cheers,
Wol