Double-Take Software

Much to my chagrin, my fellow Engineers and Developers here at Double-Take Software have taken to calling our relatively new mirroring feature the Zippy Mirror, even in Tech Support.  So though I hate calling it that, a lot of clients have begun to ask for it by name, and ask what it does.

The “Only if Source is newer” setting in the mirror options of many of our wizards and consoles can let you dramatically speed up the time it takes to perform a differential mirror to a Target device that has a copy of the data already.  This might be because you pre-staged the Target via file copies, or it could be during a re-mirror operation after a disconnect/reconnect. 

Normally, Double-Take Software products will perform a file-system block-level differential of all files in the Replication Set in order to ensure that the Target is properly quiesced and has an I/O transactionally intact copy of the data from the Source. The process can take a significant amount of time, especially if there are large numbers of small files (since we do a file difference check first).  For database servers there is no safer method to properly ensure that the Target data is intact, so if you have any form of database (including client databases like Outlook PST and OST files, Access databases, etc) you should allow the system to use the default mirror methods.

However, if you have large numbers of non-database files, then using the new Zippy Mirror will be a huge advantage to you. Zippy Mirrors involve determining the last change date/time information for each file.  If that data shows that the Source copy is older than the Target (meaning the Target is already updated), then no mirror happens for that file.  If the Source is newer, we send the file to Target.  If you have a lot of files that are less than 64K (or whatever your disk block size is), this can allow you to run through a mirror in a fraction of the time a block-level difference mirror would normally take.  We’ve seen clients reduce mirror times by over 2/3 by using this technique.

Turning on Zippy Mirror simply involves clicking the “Only if Source is Newer” checkbox where it appears in the wizards you use, or in the Connection Manager for that connection in the Double-Take Replication Console. It is a Connection Level setting, and therefore different replication connections can use different mirror types. 

Be careful, however, as attempting to use this on any form of database file could cause corruption to the Target copy of the data.  The default mirror methodology is the safest for database files, so if you used the defaults you do not need to worry. You could be getting better mirror performance on non-database data, though, so don’t ignore the setting completely. This is most definitely a case where if you have any doubts, call Technical Support (http://www.doubletake.com/support) and have them help you figure out if Zippy Mirror is right for you.