Host-based replication


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Replication sizing
Companies that need to replicate data between sites must determine the right amount of storage to purchase for the remote site and how much bandwidth is required between sites. Replication software products include a variety of tools that tell how much and how often data changes, and when it changes; based on those statistics, the software recommends ideal levels of storage and bandwidth.

Double-Take Software's Throughput Diagnostics Utility simulates the replication set and makes recommendations on how to configure the replication connection. Operating in this mode, Double-Take doesn't replicate data to the target server, but instead tracks all I/O operations on the source server and estimates the amount of bandwidth needed.

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Windows host-level replication software
Click here for a comparison of Windows host-level replication software (PDF).

Double-Take also offers low, medium or high data compression levels. The appropriate level of compression for each server is determined by establishing what additional amount of overhead the server can support. Because each higher level of compression adds about 5% more overhead to the server, a server running replication and the highest level of compression could experience CPU overhead rates as high as 15% to 20%.

Softek's Replicator can be set to run in tracking or pass-through mode. In this mode, Replicator is installed on the source server, but simulates only data replication while gathering the necessary statistics to make a determination of how long replication will take. One of the parameters that Replicator monitors is the packet chunk size, which is the size of the buffer that gets sent by the source server to the target server. Softek finds that smaller packets work best when replicating data over long distances. By tuning the source server to send smaller packets, it takes less time for the source server to resend a packet in the event the packet is dropped.

Symantec's Veritas Volume Replicator Advisor (VR-Advisor) uses one of the following methods to collect data for analysis and then makes recommendations:

  1. VRAdvisor can be installed on the source server.

  2. If Veritas Volume Manager is installed on the server, administrators may use Veritas Volume Manager's vxstat command to collect data.

  3. Administrators may also collect the data using native OS command scripts provided with VRAdvisor. In AIX environments, VRAdvisor uses the lmstat command; on HP-UX and Linux servers it uses sar; and on Sun Solaris OSes it uses iostat.

V VR uses the Storage Replicator Log (SRL) to buffer writes before replicating them. The size of the SRL will vary according to whether VVR replicates data synchronously or asynchronously between the servers. VRAdvisor also lets users create what-if scenarios so they can simulate app growth by varying the parameters and then recalculating the results.

This was first published in December 2006

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