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Slow Restores


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I recently tested restoring a large volume of data (48GB, 50,000 files, 7500 folders) and the restore took around 30 hours. Obviously this is an extremely long time considering the backup takes under an hour. Does anyone know what to adjust to speed up the restores? What I noticed from watching the job is that it seemed to be searching the tape rather frequently for files which may be the source of the slow speed. Here is what I believe are the critical details.




1. Backing up to DDS4 tapes in an HP SureStore DAT40x6 autoloader.


2. Operating system is Win2K server on all boxes


3. Retrospect Version is 6.0 on all.


4. Data compression in hardware


5. Restore job type is selected files

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Obviously this is an extremely long time considering the backup takes under an hour.

It sounds like you are referring to an incremental backup - not a full volume backup. An incremental backup is only going to backup the new or changed files, taking much less time then a complete backup of the entire drive from scratch. I don't believe the drive you are using is capable of backing up a full 48 gig in 1 hour. The qaulity of the tape, the speed of the computer, the connection to the client computer (if a network restore), the quality of the tape drive and the connection between the drive and the computer - these can all play a role in the speed of your restore.




Constantly trying to read the same spot on the tape is known as Backhitching (or shoeshining). The Ecrix site (http://www.ecrix.com/tour/tour2-home.cfm) has a great definition on this, which I'm including below:




What is Backhitching?


Streaming tape drives need to keep the tape moving at a constant fixed speed in order to read and write data. The drive cannot read or write with the tape moving at any other speed. Backhitching occurs when the data transfer rate from the host is slower than the rate at which the tape needs to write data.




When this happens, the tape drive writes out all the data in its buffer, and then stops and waits for the host to send more data. But before it can start writing again, the drive backs up the tape so that it can accelerate back to its fixed streaming speed, and then resumes writing at the exact spot where writing was stopped. If the tape were to restart from this spot without backing up first, a blank section would be left on the tape while the drive accelerates back up to speed, and some of the tape capacity would be wasted. This stop/back up/start/accelerate/resume cycle is called "backhitching," or "shoeshining."




What Causes Backhitching?


There are many reasons why the host might not be able to feed the data to the drive at the required rate. For example, the data might be coming over a network that suffers congestion which interferes with the data transfer. Or, the host processor could be busy with other tasks that slow down its transfer rate.

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