Continued from Part II
Software-based synchronous: Each write to the local storage array from the host is also redirected over an IP connection to a remote host, then also written to that hosts connected storage. The benefit here is more granular control of data replication, but the distance limitation still applies. The trade-off is CPU utilization on the production host, and each host must replicate its own data. This means if you have 100 hosts on the production side, all 100 would need to replicate it's Luns to the disaster site, opposed to one storage array replicating to one storage array at the disaster site.
Software-based asynchronous: Same as hardware based above, but driven by each host. Also, very few software vendors provide for Timestamps and ID's of data to guarantee transactional consistency at the remote site in case of link failure. Again, do your homework.
As you can see, this is an advanced topic that needs to be looked at carefully. Defining the type of link and link bandwidth is a subject all in itself. There are new techniques coming to the fore shortly that will expand this subject dramatically. iSCSI and iFCP protocols, along with InfiniBand and VIA environments will make your choices even more confusing. You can outsource all this to a storage utility vendor or if you have inhouse expertise, you can do an RFP for your requirements. If not, I would suggest using a storage consultant/architect who can help you in your decisions.
To go back to Part I go to http://www.searchStorage.com/ateQuestionNResponse/0,289625,sid5_cid414545_tax286192,00.html
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This was first published in September 2001