|HOME||RADIO SURVEYS||KWFM||WHITE PAPERS|
Scheduling Replication Throttles
AIM allows you to create a Transfer Schedule for each replication destination node. The transfer schedule works by injecting delays between replication file transfers. In the screenshot, I've put together an elaborate schedule that caps replication throughout the work hours of the day, leaving it unthrottled from 8pm to 7am. The schedule is based on hours/minutes, so you have adequate granularity to control the bandwidth utilization.
As the throttle works by injecting delays between the file transfers, the replication will be somewhat "bursty". You can control the length of the bursts by increasing or decreasing replication file sizes; you can control the frequency of the bursts by increasing or decreasing the delay.
Automating DR Site Tasks With Custom Markers
In addition to Snapshot Markers, AIM includes a Custom Marker facility that can be used to trigger housekeeping scripts or programs at the DR site. Custom Markers can be as simple as a Windows command file or VB script that takes a snapshot and triggers a tape backup.
You can have multiple custom markers for different actions, such as taking a nightly incremental, a weekly full backup, or sending a weekly storage utilization report via email.
Volume Consistency Groups
AIM has a facility to create volume consistency groups for inter-dependent volumes. With AIM Groups, a Group Snapshot or Group Custom Marker will execute against all the volumes in the group at the same point in time to assure consistency is maintained.
This is particularly useful, for example, when replicating an Oracle database whose tables span multiple volumes. With an AIM Group snapshot, you would have what some vendors call "Storage-based Consistency" (as opposed to Application- based Consistency achieved by stopping or quiescing the application, flushing file manager and I/O subsystem caches, etc.) The set of volumes would be in a state consistent with having experienced a system crash or power failure.
From a purely logical point of view, so-called Storage-based Consistency does not guarantee that the set of volumes will indeed be in an inter-dependent content consistent state, because the Storage Processor has no control over the caching performed by the host OS that the application (in this example, an Oracle database) is running on.
Example of Snapshots on Inter-Dependent Oracle Volumes
Nonetheless, placing inter-dependent volumes in a Consistenty Group such as that offered by AIM improves coherency of snapshot on the set and also facilitates scripting as you'll only be issuing a command once across the group instead of multiple times.
Finally, AIM Group commands can be pushed from the host owning the volumes, which can be used to flush application server cache, take the snapshot, thus further improving content consistency over the volumes in the set.
AIM can replicate any SANmelody or SANsymphony virtual volume, including snapshots. We can use this functionality to de-duplicate the replicated data with respect to our RPO.
Consider a volume "Test" which we have mapped to an application server, perhaps a SQL or Exchange server. We use the Snapshot Manager in SANmelody or SANsymphony to create a snapshot volume called "Test-SS". We make the snapshot a "Complete Image" or "Clone" (or BCV, if you prefer) of our "Test" volume.
We create a snapshot script on our SQL or Exchange server to refresh or increment our snapshot every 15 minutes. Thus every 15 minutes, our Test-SS clone volume will be updated with the most recent changes to the Test volume.
A Complete Image Snapshot
Now we make our Test-SS volume the source of an AIM replication to the DR site. Every 15 minutes, as the snapshot increments, those latest changed blocks will be copied to the snapshot's virtual volume and so, as writes, they will be replicated to the DR site. We have, in effect, de-duplicated any multiple writes that may have occurred on any given block during the past 15 minutes.
A Complete Image Snapshot... Replicated
How useful is de-duplication in async replication? That all depends on the application and the server's file system that owns the volume you are replicating. If, when re-writing a file, the file system re-writes the same blocks, then de-duplication can pay off handsomely. This behavior would typically be the case with large files such as database tables where specific records occupying specific blocks on the volume may be written multiple times.
However, in the case of office productivity applications like word processors, it is not a given that de-duplication will have much value. As a software engineer frequently working on a text editor, I developed the habit of frequently hitting "Control-S" (in fact, it was an "Apple-S"...) to make sure I didn't lose my work. So how does a given word processor work? Each time I hit "Save" is it writing a new copy of the file before deleting the previous saved copy? And so the file system writing therefore writing the new file into different blocks? If so, any of the industry's block-level de-duplication technologies isn't going to work. As the application is also doing file renames during the saves, chances are most of the file-level replication packages won't be able to de-duplicate those files, either. C'est la vie.
Take A Snapshot Every 15 Minutes
In addition to automated mechanisms available with the AIM GUI, AIM includes a command line interface that can be driven by script from the hosts or from the DataCore storage controllers.
A Windows script to invoke a snapshot from the hosts can be all of 2 or 20 lines, depending on how elaborate you want to get. In the example above where we periodically replicate source snapshot updates, the script I used was all of 5 lines, including the "@echo off".
The script was installed as a Windows Scheduled Task, set to run every 15 minutes. Similar scripts can easily be run as cron jobs in Unix implementations.
To be completed...
The AIM Client package from DataCore allows you to replicate individual Windows servers, desktops or laptops back to a SANmelody or SANsymphony storage server. The product is ideal for remote or satelite offices where the cost of a SAN storage array is unjustified. With AIM Client you can integrate smaller satelite offices into an overall DR plan without great expense.
The AIM Client GUI
Examples of implementation forthcoming... it all takes time, folks... If you would like to see this now, give me a call and I'll set up a webinar and demonstrate the product. AIM Client is an elegant solution.
Whether or not you currently use Thin Provisioning in your environment, deploying it in the context of a DR or Async Replication project will save you a LOT in both time and money. As you will have noted in the these discussions, async replication means double the storage. Some of the techniques we have discussed, such as snapshot clones or BCV volumes to implement de-duplication further gobble up disk space.
With Thin Provisioning, you can effectively reduce disk consumption while still taking "Complete Images" or BCVs of your volumes. Imagine you have a 300GB VMFS volume based on Thin Provisioned storage. Suppose it currently has 125GB of data on it, 125GB of allocated storage. Suppose you wish to use our aforementioned de-duplication technique on this volume. You create a "Complete Image" snapshot of the volume onto Thin Provisioned storage: the BCV or clone will weigh in at 125GB, but can grow to 300GB as necessary. You use AIM to replicate the snapshot... you replicate 125GB, not 300GB. Your actively replicated AIM destination (or DR) volume, also based on pooled, Thin Provisioned storage, weighs only 125GB, but can grow to 300GB. You take a snapshot of the replicated volume so you can test your DR or run a backup. The Thin Provisioned snapshot begins its life as an empty disk geometry, weighing all of one "Storage Allocation Unit", perhaps 8 or 16MB. As replication activity continues and new blocks arrive and are destaged into the DR volume, the snapshot volume will slowly grow.
Thin Provisioning's Storage Pooling aspect means you won't be spending hours studying how you're going to allocate partitions for the replicated volumes on the DR site's storage arrays. DataCore's Thin Provisioning manager will handle all that for you.
For a complete discussion of Storage Pooling, Thin Provisioning and Over Subscription, please read my white paper.