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date: Fri, 9 May 2008 09:11:04 -0700,
group: microsoft.public.exchange.admin
back
Re: Exchange 2007 Storage Calculator
It doesn't, but I'm not quite sure I'd agree that it couldn't be done.
For VSS snapshot providers, you have three basic types:
1. Clones
2. Copy on write
3. Allocate on write
The first uses 100% of the usable space of the LUN that is backed up. The
other two use space equal to the change delta and a small amount of space
for snapshot metadata. If you archive or "preserve the state of the
transaction logs" as described in kb822896 and
http://msdn.microsoft.com/en-us/library/bb891802(EXCHG.80).aspx for Exchange
2007, then you have to consider the archived logs as well. The main
differentiator beween CoW and AoW is performance. For CoW, when a snapshot
is in place and you overwaite a block you have to read it, write it to the
difference area, then overwrite (penalty of 3). For AoW, you never
overwrite in the first place so it's just one write operation (no penalty).
In designs that rely on CoW, you want to minimize the number of snapshots in
place at any given time to minimize the performance penalty. With AoW (the
Netapp hardware provider is an example), you don't have a performance
penalty and designs typically have many snapshots retained.
I guess the problem with defining the space consumed for CoW or AoW is that
the change delta is a fairly unpredictable thing. It's sort of like Quantum
Physics. The best approach in my opinion is a statistical one; you collect
data over a long period of time, normalize a curve, then add enough standard
deviations to reach the desired level of reliability.
John
"Mark Arnold [MVP]" wrote in message
news:g23924tcvphr2lsl04luvvintu3nklm4h2@4ax.com...
> On Fri, 9 May 2008 09:11:04 -0700, RJ
> wrote:
>
>>When using the Exchange 2007 storage calculator does the tool take into
>>account the amount of disk space that is needed to do backups? For
>>example
>>if you wanted to do a VSS solution would you need to add on the extra
>>diskspace to the number that is calculated for storage requirements?
>
> It doesn't change and I don't think Microsoft could hope to achieve it
> in a way that would be satisfactory for the users. Would they design
> it based on WAFL? Would they design it based on Copy on Write?
> If they made a choice, the latter, say, as it's the common offering,
> they'd have to ask you about what software and hardware you'd propose
> to use because each of them does their own thing and affects both
> performance of the storage and number of backups that can be
> maintained.
> It would probably make sense for them to plug in some numbers based on
> their DPM offering, something that gets pushed when CCR is touted.
date: Fri, 9 May 2008 11:04:41 -0700
author: John Fullbright fjohn@donotspamnetappdotcom
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