Jump to content

Client backup sits idle while tape runs

Recommended Posts

It happens on different clients on different occasions. In the morning, when we show up to the office, Retrospect is asking for a new tape because it has run to the end. I know what we're backing up doesn't come near to the capacity of the tape (And we're running hardware compression). It's like Retrospect is waiting for something, but what? And it doesn't happen all the time.




Below is a portion of the log and you can see towards the end where it took 9.5 hours to copy 2.2 gigs. And it actually stopped (?) or went idle while the tape ran.




Any one seen this before?




- 5/21/2002 10:01:29 PM: Copying PrintSmith Folder on Tony's iMac


5/21/2002 10:01:29 PM: Connected to Tony's iMac


5/21/2002 10:02:35 PM: Execution completed successfully


Completed: 602 files, 41.9 MB


Performance: 38.6 MB/minute


Duration: 00:01:06 (00:00:01 idle/loading/preparing)




- 5/21/2002 10:02:35 PM: Copying System Folder on G4/450 BARB


5/21/2002 10:02:35 PM: Connected to G4/450 BARB


5/22/2002 7:35:45 AM: Execution stopped by operator


Remaining: 472 files, 2.0 GB


Completed: 3793 files, 478.5 MB


Performance: 56.6 MB/minute


Duration: 09:33:10 (09:24:43 idle/loading/preparing)




5/22/2002 7:35:45 AM: Execution stopped by operator


Total performance: 66.4 MB/minute


Total duration: 12:35:32 (09:25:14 idle/loading/preparing)

Link to comment
Share on other sites

From the Knowledgebase:






Why are my tapes filling up sooner than I expected?




For typical everyday use, when your tape is full, it may store up to 30% less data than its ideal maximum capacity.




If you use a drive with hardware compression, you can effectively increase the capacity of your tapes. Your tape's actual capacity will depend largely upon how well the data you are copying compresses. Text compresses well, for example, but applications do not.




If you back up many small files, your tape's actual capacity will also decrease. If the amount of data that you are getting on your media does not appear to reflect compression, it could result from several factors. Often, compression is working, but total capacity is decreased due to media, device or communication problems, or other factors. Tape capacities are discussed in detail in your User's Guide and in other Knowledgebase records.




Retrospect requests a new tape for one of three reasons:




- The tape drive reports the current tape is full.




- An error occurred while writing to the tape. Open the log to see if an error occurred.




- You selected Skip or Missing while configuring a backup set, or you are performing a New Media backup.




See also Tech Note 305, Streaming Tape Capacities Explained, in the Knowledgebase or at www.dantz.com/support.






Tech Note 305






for the Macintosh or Windows




Tech Note No. 305


Streaming Tape Capacities Explained


One of the questions most often asked of Dantz is "Why do my tapes fill up sooner than expected?" This technical note explains the varying capacities of streaming tape drives.




Influences on Capacity


Streaming tape capacity varies, sometimes dramatically, because of the nature of the drives. Unlike block-addressable devices which use formatted media of a known capacity, a streaming tape drive formats the tape as it is written. Following are the five factors that influence streaming tape drive capacity.




1. Tape Condition


Streaming tape drives automatically verify data by reading each frame of data immediately after writing it. If the data does not verify, the tape drive moves to the next part of the tape and attempts to write the frame again. As long as the drive successfully writes the data before trying too many times, it will not report a media failure and the backup will continue. This error rewrite process, which can be caused by dirty heads on the tape drive, deteriorating media, temperature changes, or a host of other conditions, reduces the effective capacity of the tape.




2. Media Failure


If a tape fills up sooner than expected, your first step should be to look in the operations log to see if any media failures (error 206) were reported. It is possible that Retrospect encountered a failure on the tape, which forced it to prematurely stop using the tape (before it was filled) and request another tape to continue the backup. (See Dantz Technical Note #306, Recovering from Error 206 Media Failure.)




3. Speed of the Source


The speed of the source volume is the single most important factor in determining streaming tape capacity. Each time the buffer in the drive runs out of data to copy, an "underrun" occurs and unused tape streams on by, wasting raw tape capacity. The more underruns, the greater the reduction in capacity.




A fast hard disk on the same Macintosh as the backup drive tends to result in fewer underruns and therefore allows the tape to hold more user data. Copying files over a network tends to be slower, resulting in more underruns and lower capacities, especially over LocalTalk.




The size of files being copied has a dramatic influence on the source speed. The same computer and hard disk may take four times as long to transfer 100MB of small files as it does to transfer a few large ones also totaling 100MB. The speed of the computer matters, too. A computer with a fast architecture (which includes the CPU among other things) may transfer files at twice the rate of a slower computer, even though the hard disk and file sizes are the same.




4. Hardware Compression


In order to boost capacity and speed, many streaming tape drives contain a chip to compress the data coming into the drive before it is written to tape. The amount of extra capacity achieved, if any, is dependent upon how well the incoming data compresses, and this varies widely. Here, from best to worst, is a listing of approximate file compressibility: database, text, spreadsheet, graphics, applications, System files, previously compressed files. Some files may reduce to a fraction of their original size while others may not diminish in size at all.




It is impossible to predict how much compression will be achieved; you cannot know until the data is actually copied to the tape drive. For display purposes, Retrospect assumes that using hardware compression doubles capacity, which is somewhat optimistic with certain types of files. A 50% increase in capacity is a more conservative rule of thumb. However, we must emphasize the broad variation that makes any prediction suspect.




5. Tape Length


As tape technology improves, so does the ability to increase the length of the media without compromising quality or capacity. DDS-2 and -3 DAT drives require specially formulated 120 or 125 meter tapes in order to write in the those formats. The latest technology DAT drives can also use the shorter tapes in the less dense DDS format, but at greatly reduced capacity.




Capacity Display


Retrospect estimates tape capacity to aid you in managing backups. No matter what tape capacity is assumed for display, Retrospect continues to write to the very end of the tape (unless a media failure occurs). To see Retrospect's estimated capacity of a tape, go to the StorageSet configuration window. In its Summary tab, the assumed tape capacity is shown next to "Available."




Used: 2.9 G for 28,977 files


Available: exceeds estimated capacity of 2.6 G


Storage: 1 member, 83 sessions, 9 SnapShots




Options: hardware compression


Security: none


Catalog: Startup Drive: System Folder: Preferences: Retrospect: Catalogs: Katzenjammer




Because DAT drives usually do not tell Retrospect what size of tape is inserted, and for all the reasons discussed above, Retrospect's assumptions of tape capacity may be inaccurate. Experience may indicate a more accurate number, at which time you may change the capacity display by clicking the Set Capacity button in the StorageSet configuration window.




Actual Results May Vary


Do not be surprised if your tapes end up with less than their advertised capacities. Some tape drives are represented as being capable of higher capacities than the drives normally achieve in day to day use. The representations refer to the amount of data before it gets compressed by a tape drive with hardware compression capability-and they often assume generous compression rates. By monitoring the capacity of just a few of your tapes of typical backups, you can know what capacity to expect from the rest of your tapes.

Link to comment
Share on other sites


This topic is now archived and is closed to further replies.

  • Create New...