linux-ext4 - Re: How to use mkfs.ext4 "stride=" on RAID correctly?

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Message-ID: <fa7b26051001131658ya96c3b0ieb6010c6fba389c4@mail.gmail.com>
Date:	Wed, 13 Jan 2010 18:58:46 -0600
From:	Mike Mestnik <cheako911@...il.com>
To:	linux-ext4@...r.kernel.org
Subject: Re: How to use mkfs.ext4 "stride=" on RAID correctly?

On Wed, Jan 13, 2010 at 6:13 PM, Mike Mestnik <cheako@...i.com> wrote:
> What should this value be?  From what I gather it should be the length
> of data stored on a single disk for each RAID level block.  If that's
> the case how is it that two given data blocks are calculated to be on
> separate drives?  It seams to me that the stripe-width is also
> essential in this regard, but the man page does not reflect this.
>
> For example let's say that stride=1, then which of the following
> blocks are not on the same drive as 1: 8 9 10?
> The answer is dependent on the number data disks, like so.
> Where x = n - 1 or n depending on the RAID type.
> if x = 2 then 9
> if x = 3 then 8 and 10
> if x = 5 then 8 and 9
>
Wait!!
I got this all wrong, one would need all of x, n, and stride to
successfully determine the disk used for a given stride.

Seams to me mkfs is missing some parameters.  What about [-g
blocks-per-group]...

http://tldp.org/HOWTO/Software-RAID-0.4x-HOWTO-8.html

> There is no way to make this calculation with out knowing x, further
> more calculating x based of of both stride and stripe-width is round
> about...  Why not simply ask for x, the number of data disks and have
> stripe-width be the value that is calculated, as stride might not go
> into stripe-width evenly leaving you with a headache.
>
> Did I locate a bug?
>
> Is there a better forum for this discussion?
>
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