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Limitations of MS SQL 2008 mirroring vs EMC SRDF synchronous SAN replication

I am trying to get a reliable/consistent answer as the first attachment seems
to indicate EMC endorse MS SQL mirroring (though my EMC sales person is
strongly proposing EMC SRDF over MS SQL DB mirroring, saying that SRDF
is more reliable than MS SQL mirroring & he quoted a few reference sites
using SRDF for MS SQL DR recovery with zero RPO

Attachment 2 is a product DoubleTake which mentions that synchronous
SAN replication has a distance limit of 10 miles (about 16 km)

However, link below (from EMC) mentions the WAN (fibre link, guess it refers
to dark fibre though our EMC Sales said dark fibre is not essential, copper
IP link suffice as our current link is copper) has a max of 300km.
  http://www.netcraftsmen.net/component/content/article/69-data-center/1204.html

Q1:
so who is right?  Is it 10 miles max distance or 300km?

Q2:
My DBA colleague mentioned that SQL DB mirroring also has a max distance
limit (for WAN) but some internet links out there says there's no limit.  Eg:
  http://social.msdn.microsoft.com/Forums/en-US/sqldatabasemirroring/thread/ae3c2a7b-c127-4e68-a9ca-0f327e8b0259
  So who's right & what's the distance limit for MS SQL mirror if any?

Q3:
I seem to read somewhere (but lost track of that url/link) that for MS SQL DB
mirroring (synchronous), certain tables (like tempfiles or tempspace ?).  Can
someone list out all the tables that can't be mirrored & are these tables crucial?

Q4:
Besides cost, appreciate if anyone can share any other comparisons, pros/cons
of both technologies as finding it hard to decide.  If anyone has done Oracle
DB synchronous mirroring before, can share how you feel about it.  Any
SRDF replication experience are most welcome as well.  It costs almost
US$100k more here to have SRDF.
h6393-EMCBCP-MSql-db-mirroring-c.pdf
doubltake-SQLServereplicatnSAN-W.pdf
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sunhux
Asked:
sunhux
1 Solution
 
sunhuxAuthor Commented:
Also, can you throw some light on security:
heard that for MS SQL, need to configure PKI.
What about SRDF?  What kind of 'encryption'
over the WAN link comes bundled with SRDF?
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arnoldCommented:
I've not looked at it recently, but when I did the SAN level copy versus the application level copy which is what sql mirroring does are two different.
I.e. the SAN level copy the sql server on the other side can not be running.
When a failure occurs on the primary, the backup/secondary sql server is started up at which point it loads the data from the "local" SAN and starts its operations.

Mirroring deals with application level data transfer such that both sql server instances are always up and running while one is the principal and the other is a mirror that unless you have enterprise version can not be accessed (enterprise version you can create a snapshot on the mirror side and access the data in read-only mode).

If you have automatic transition, mirroring would be faster, not as fast as a cluster.
Mirroring requires that the applications that use the database, are mirroring aware.
The distance issue relates to the witness which triggers the transition from one to the other i.e. whether bandwidth constraints could lead to the witness interpreting a non-response because of network saturation as principal down, triggering the transition to the mirror. Or the principal due to network saturation looses contact with both the witness and the mirror leading to a lack of quorum which will bring down the principal instance.

Instead of looking at the available options, what is the requirement for the application?
i.e. do you have geographical location and need the resource all over the place?
merge/transactional replication might be an option
http://msdn.microsoft.com/en-us/library/ms151198.aspx
http://msdn.microsoft.com/en-us/library/ms151329.aspx

This way each location has a local instance which speeds up access, while also makes data added/changed locally available to the remote with "minimal" delay.
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