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тАО11-25-2008 06:49 AM
тАО11-25-2008 06:49 AM
I was just poking around in the VMS help on "Set Shadow /Site", and I came across the following which I thought was kinda interesting...
....In a Fibre Channel configuration, shadow set members at different sites are directly attached to the system. For the Volume Shadowing and cluster software, there is no distinction between local and remote in multiple-site Fibre Channel configurations....
Does this mean that the /Site qualifier is ineffective in concentrating "Read" requests to the shadow units in the Primary Data Center.
Note:: This question is specifically about the qualifier. I appreciate that simple latency will cause the majority of the reads will be satisfied locally.
Dave.
Solved! Go to Solution.
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тАО11-25-2008 07:21 AM
тАО11-25-2008 07:21 AM
Re: Shadowing between data-centers.
do the math.
>>>
hosted in a different data center ~3 miles away
<<<
Sorry, I have to switch to metrics.
3 miles (FIBRE distancw, I will assume !!) ~ 5 km. ~ 10 km round trip.
Speed of light in glass = 200,000 KM/SEC =>
round trip = 1/20,000 th of a second ; or much less then the loss within any device etc.
This means, that for practical purposes your sites can still be considered "adjacent"
hth
Proost.
Have one on me.
jpe
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тАО11-25-2008 07:24 AM
тАО11-25-2008 07:24 AM
Re: Shadowing between data-centers.
I hit Sumit too fast.
So, the conclusion is, that in your situation, the SITE mechanism has no noticeable added value.
Proost.
Have one on me.
jpe
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тАО11-25-2008 07:26 AM
тАО11-25-2008 07:26 AM
Re: Shadowing between data-centers.
Please note my "NOTE". Math is not the issue, the qualifier is.
I am simply asking whether the /site qualifier is meaningful in a SAN fabric context. The fact that my storage subsystems are only 5KM apart is irrelevent, it could be 50, 500, or 5000km,
the question about the /site qualifier remains the same.
Thank you.
Dave.
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тАО11-25-2008 07:32 AM
тАО11-25-2008 07:32 AM
Re: Shadowing between data-centers.
but that is the reason that you would use this qualifier.
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тАО11-25-2008 07:34 AM
тАО11-25-2008 07:34 AM
Re: Shadowing between data-centers.
Dave.
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тАО11-25-2008 07:42 AM
тАО11-25-2008 07:42 AM
Re: Shadowing between data-centers.
For shadowing it makes no difference where the disks are. If you indicate that it must concentrate the reads on 1 disk (via /site), it will do so. Even if it's the slower one (e.g. far away).
Wim
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тАО11-25-2008 07:56 AM
тАО11-25-2008 07:56 AM
Solutionthe intention of this sentence should be to point out, that the volume shadowing or cluster software can NOT distinguish between 'local' and 'remote' disks, so you may want to use the /SITE qualifier to indicate, which disk is at which site.
See a recent 'OpenVMS Shadowing in Action' presentation (page 51 ff.) from Keith Parris:
http://www2.openvms.org/kparris/hptf2005_VolShad.ppt#500,1,HP OpenVMS Volume Shadowing in Action
Volker.
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тАО11-25-2008 07:57 AM
тАО11-25-2008 07:57 AM
Re: Shadowing between data-centers.
Traditionally, SHADOWING will issue the READs only to the drives attached to the local system.
And here is the catch: to VMS _ALL_ FC disks are "locally connected".
And that would defeat the read performance advantage of reading only locally __if the time to transfer the request and the answer grow to significant amounts__
As a way to resolve that, the SITE mechanism (and the related COST mechanism) were introduced.
It is thereby possible to tell VMS that a node and a specific drive are co-located, or not, and so re-introduce the read-locally-only principle.
And the value only applies at "significant (in transfer time) distances"
hth
Proost.
Have one on me.
jpe
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тАО11-25-2008 08:21 AM
тАО11-25-2008 08:21 AM
Re: Shadowing between data-centers.
i agree it could have been clearer i.e
" ...In a Fibre Channel configuration, shadow set members at different sites are directly attached to the system. For the Volume Shadowing and cluster software, there is no distinction between local and remote in multiple-site Fibre Channel configurations.... so you may wish to use this setting to concentrate read requests on local fc disks"
:-)