Data Mirroring and Extension
June 4, 2011Data Mirroring and Extension
Options
1) Remote data mirroring extender chassis
2) ESCON channel interface
3) On-channel compression for ESCON interface
4) 0C3 and T3 network interface
5) SNMP-based monitor and control system
6) Identically configured redundant system or redundant chassis with spare cards
7) Vendor-supplied cabinet
Why Buy?
Data mirroring copies production data on Direct Access Storage Devices (DASD) or RAID (Redundant Array of Independent Disks) from one location to another. When mirroring occurs in real time, information at the second site is always a mirror image of information at the production site. If a disaster happens, operations can be restored very quickly from the mirror-image disk. Companies acquire data mirroring extenders to increase the distance between the primary and the recovery site, which enhances disaster protection. Remote mirroring’s impact on production application response time is a function of the distance between the primary and recovery site; however, the right data mirroring extender can send information faster or farther while minimizing impact to response time.
What To Look For
1) Hardware-based system designed specifically to extend remote data mirroring
2) System latency between 35 and 50 microseconds
3) On-channel compression for ESCON channel interface
4) Bi-directional network support so the extender can send and receive data at the same time
5) Open mirroring support so the extender works transparently with any data mirroring unit that has an ESCON channel, without any software or hardware changes
6) Built-in diagnostics for the network interface
7) SNMP MIB and HP OpenView for complete SNMP-based monitor and control system
8) Text-based monitor and control system for faster remote access
9) Rack-mount chassis available in vendor cabinet or customer cabinet
10) Full configuration, implementation, and testing assistance with on-going 24×7 support
11) Secure web-based access to vendor’s customer support database for status of support issues and other support information
12) Certification or validation testing with storage vendors’ remote data mirroring products
13) Base system that includes two ESCON channels, two network interfaces, a monitor, and control system
Cost Considerations
A detailed cost analysis will often reveal hidden expenses. When adding up costs, consider the following:
1) Charges for the DASD or RAID mirroring
2) License fees for the software that resides on the RAID or DASD control unit
3) Price of the base remote data mirroring extender and options included in this charge
4) Price for additional options not included with the base remote data mirroring extender
5) Telecommunication costs and savings from On-Channel Compression
6) D/R vendor fees remote data mirroring service purchased from a D/R vendor
7) Installation fees
8) Shipping and handling fees
9) Daily operation expenses
10) Additional testing fees
11) Charges for on-going maintenance and customer support
Be Prepared
In order to bid for your business, vendors will want to know:
1) The data mirroring product you have installed or will be installing
2) The distance between your primary site and recovery site
3) The wide-area network between your primary site and recovery site
4) The mode of mirroring you plan to use or are using (synchronous, asynchronous, semi-synchronous)
5) The read/write ratio of the data you are or will be mirroring remotely
6) Pre-production, production, and post-production testing requirements
Key Questions To Ask Potential Vendors
1) Is the data mirroring extender designed specifically for the remote data mirroring application?
2) Is the data mirroring extender’s latency 50 microseconds or less?
3) Does data compression occur directly on the channel interface, before the channel extender’s CPU receives the data?
4) Can you provide a dose or exact estimate of how much bandwidth the application will require?
5) Can the data mirroring extender be reconfigured to work with a new mirroring product on the fly?
6) Can the data mirroring extender send and receive information simultaneously?
7) Is the extender certified with the storage vendor’s data mirroring product?
8) Will a disaster recovery provider be a reference for the vendor’s customer support?
9) Is there a storage expert on staff with experience sizing and configuring customer storage applications?
10) What does the warranty cover?
Key Questions To Ask Vendor References
1) How long have you contracted with the vendor?
2) Why was this provider chosen? Were others considered?
3) What is your response time requirement for the remote data mirroring application in synchronous, semi-synchronous, and asynchronous mode?
4) What compression ratios are you achieving?
5) Vendor strengths, weaknesses?
6) What is your experience with customer support?
Buyer Beware
1) A data mirroring extender that does not use a hardware-based design specific to remote data mirroring extension may have a lot of software overhead. This extra processing time can increase the impact of remote mirroring on production application response time.
2) Data compression maybe implemented on network interface or within the chassis instead of on the ESCON channel Link-based and internal compression will not generate powerful compression ratios so you may not save as much bandwidth as possible.
3) The data mirroring extender may not be able to send and receive data simultaneously. If so, you will need a separate communications link for each mirroring system, which will increase communications costs dramatically.
4) The data mirroring extender may not really work transparently with any ESCON-based mirroring stem, requiring on-site support and downtime to reconfiguration if you change or add a different mirroring system.
5) If the base chassis does not include two ESCON channels, two network interfaces, and a system monitor at no extra charge, you maybe overpaying.
What Next?
1) Request a presentation from all vendor finalists at your company’s location.
2) Seek written follow-ups to questions not answered during the vendor presentations.
3) Request a head-to-head test for vendor finalists.
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