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RAID 1 Mirror From Two SMB Devices?



 
 
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  #1  
Old February 5th 07, 12:37 AM posted to comp.arch.storage
Will
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Posts: 338
Default RAID 1 Mirror From Two SMB Devices?

Does any vendor sell software for Windows that would make a RAID 1 mirror
from two SMB NAS devices on the back end? Performance on NAS has gotten
so good with gigabit ethernet that there is little reason to prefer SANs for
most applications. What I would like to remove from the equation is the
impact from a hardware failure of the NAS box itself.

The built-in Windows Server mirroring, and the more advanced version of the
same sold by Veritas, only want to work with devices that are seen as
locally attached. I want to know if anyone makes similar functionality
for SMB connected NAS devices. Having the same capability over the NAS
abstraction would be quite powerful and useful.

--
Will


  #2  
Old February 5th 07, 03:36 AM posted to comp.arch.storage
Bill Todd
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Posts: 162
Default RAID 1 Mirror From Two SMB Devices?

Will wrote:
Does any vendor sell software for Windows that would make a RAID 1 mirror
from two SMB NAS devices on the back end?


That would be a technical impossibility: NAS operates at the file
level, while RAID operates at the disk-block level.

Now, it would be possible to create a local file system that sent
identical file-level updates to two NAS back-ends (and indeed I think
that at least one of the recent 'file virtualization' start-ups does
something of that sort, though using an intermediary rather than a local
file system as the fork point - for reasons that may become obvious below).

Performance on NAS has gotten
so good with gigabit ethernet that there is little reason to prefer SANs for
most applications. What I would like to remove from the equation is the
impact from a hardware failure of the NAS box itself.


It's not clear what you think you'd be gaining. First of all, you
couldn't share the data on the NAS back-ends with other clients, because
(by definition) the single client controlling the mirroring between them
has no way to coordinate such sharing with others. So what you'd have
is a client, using a local stub file system, to control two more private
copies elsewhe why not just let the client file system directly
control disks elsewhere (or, for that matter, locally-mirrored disks:
if the client goes down it won't matter that the data remains accessible)?


The built-in Windows Server mirroring, and the more advanced version of the
same sold by Veritas, only want to work with devices that are seen as
locally attached.


The issue is not local vs. remote, it's file-level vs. block-level access.

I want to know if anyone makes similar functionality
for SMB connected NAS devices.


Almost certainly not, since the possibilities for corruption (unless the
NAS back ends where stringently protected from any access by other
parties) would be rampant. The closest you're likely to find is a
proprietary file system that uses proprietary protocols to access closed
back ends: there are actually a couple of reasons one might want to do
that, though hardly compelling ones (i.e., it might fall out as a useful
side-effect of an implementation, but isn't something one would go out
of one's way to implement).

Having the same capability over the NAS
abstraction would be quite powerful and useful.


Not really. It does provide a level of device independence, but at the
cost of a couple of additional levels of software (plus additional
overhead ensuring that the back ends remain in sync: that's relatively
easy using idempotent block writes compared with what's required to
handle more complex logical updates).

- bill

 




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