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From: Sam <sah@softcardsystems.com>
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To: "Greg 'groggy' Lehey" <grog@freebsd.org>
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cc: Sven Willenberger <sven@dmv.com>
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Subject: Re: Creating a striped set of mirrors using gvinum
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> On Saturday, 19 March 2005 at 17:58:06 -0500, Sven Willenberger wrote:
>> I am hoping someone has found a way to create this type of raid set
>> using [g]vinum. I see that it is a trivial matter to create a mirror of
>> 2 striped sets but I have not seen a way to create a stripe set out of
>> multiple mirrored sets (e.g. stripe across 3 sets of mirrors). Has
>> anyone managed to implement this and, if so, what does your
>> configuration file look like?
>
> No, it doesn't work that way.
>
>> If not, could this be added as a feature request for gvinum?
>
> To do this you should enter a PR.  It would help if you could explain
> why it's preferable to mirrored plexes.

It's preferable because:

1. After a single disk failure the rebuilding process only needs to
rebuild one disk, not a whole stripe of disks.  This also means
that if your mirror code distributes reads to the mirrored disks
for performance optimization, you don't lose that performance
for the whole array when only one disk goes out of commission.

2. You can take multiple disk failures, one per mirror.  In a mirror of
stripes you can lose two before you're sunk -- the rebuild time of having
to remirror an entire stripe on single failure means an even shakier 
bridge.

This is to say that these are the benefits in other disk systems I've 
used.  Spare management becomes more complex, unfortunately.

sah