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AIT capacity (nowhere close!)


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I have been running Retrospect 4.2 (w/ ADK 1.7) for some time on a

 

PPC 7500 running MacOS 9.1, backing up to a Sony SDX-500C AIT2 drive.

 

 

 

A few backup sets ago, I started getting only ~14GB capacity on the

 

AIT2 tapes, as opposed to the more ususal ~45GB. After running another

 

new media backup, the first tape got 45GB, but the next tape was back

 

to 14GB. A new media backup last night popped out at 14GB.

 

 

 

Anyone have any idea why this is happening, or what might have caused

 

this to occur after running successfully for more than a year? Is it

 

possible to have gotten a bad batch of tapes? That's about the only

 

thing I can think of...?

 

 

 

--

 

Richard Braun

 

braun+retrospect@cs.duke.edu

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Check out these great Knowledgebase Articles on Tape Capacity:

 

 

 

1. Tape Capacity FAQ

 

www.dantz.com/index.php3?SCREEN=knowledgebase_article&id=915

 

 

 

2. Tech Note 305 Streaming Tape Capacities Explained

 

www.dantz.com/index.php3?SCREEN=knowledgebase_article&id=530

 

 

 

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.

 

 

 

Tested Capacities

 

We have performed extensive testing of DAT drives, and our experience is summarized in the following table. Note that these are typical ranges, and that it is perfectly normal, and expected, that any particular experience may be outside these ranges in either direction.

 

 

 

Uncompressed DAT Tape Cartridge Capacities

 

Backup Source 60m DDS 90m DDS 120m DDS-2 125m DDS-3

 

local hard disk 900 to 1250 1350 to 1850 3000 to 3700 9400 to 11500

 

client over LocalTalk 600 to 1000 900 to 1500 2800 to 3400 8800 to 10600

 

client over Ethernet 800 to 1100 1200 to 1600 2900 to 3600 9100 to 11200

 

 

 

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.

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  • 3 weeks later...

An addendum to my original post (and why I still think something

 

is strange here).

 

 

 

The problem seems to be only w/ SDX2-50C tapes, the 230m, 50G native

 

capacity tapes. I ran backups on the Mac Retrospect mentioned in the

 

original post, this time using SDX2-36C tapes (170m, 36G native), and

 

got a capacity of ~45G, much closer to what I would expect. The next

 

tape I used was another SDX2-50C, and it filled at ~14G.

 

 

 

I tested the SDX2-50C tapes on a Windows Retrospect (v5.11) setup and

 

found the same thing. The larger-capcity SDX2-50C tapes filled at ~17G

 

while the SDX2-36C tapes were routinely getting over 40G capacities

 

for the same backup sets.

 

 

 

I think there is clearly a problem here that cannot be explained by

 

the vagaries of streaming media. Has anyone seen anything like this?

 

I certainly don't want to be buying a higher-priced media that has 1/3

 

the capacity in practice (AIT tapes are still rather expensive), but

 

I wonder if it is something specific to my batch of SDX2-50C (I have

 

only one sample I've looked at) or if it could be a Retrospect problem.

 

 

 

-- Richard Braun

 

 

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Retrospect streams data to the drive until the drive indicates that the tape is full. The software doesn't actually have a way of determining the actual size of the tape you are using. Regardless of whether you are using 170m or 230m, the data is sent exactly the same way.

 

 

 

That this was previously working, and is now failing on two platforms, would indicate a problem somewhere along the hardware bus. This could include tapes, cables, termination, or the drive itself. The key is to start ruling out the different variables to isolate the root cause of the problem. The most likely culprit is the tape - when they start to go bad, decreased capacity is definitely a symptom.

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That Retrospect cannot determine the actual size of the tape was my

 

understanding from previous communications w/ Dantz (quite some time

 

ago). I don't think any software I have ever seen does though, they

 

all estimate.

 

 

 

In my test, the two setups I used to test the tape capacities were

 

completely separate: separate machines (w/ different OSes, of course),

 

separate cables, separate AIT drives. That would leave the only

 

common factors the Retrospect software (which are different OS-releases

 

as well as different releases) and the AIT media itself.

 

 

 

The AIT media were purchased new and were being used in the backups

 

for the first time. If it were only one tape displaying this behavior,

 

I would definately suspect the tape. But on the MacOS setup, I have

 

several backup sets, of several members each, where the tapes fill

 

right at ~14GB (the Windows test w/ the SDX2-50C tape filled at somewhat

 

higher capacity ~17GB, but I only tested one tape w/ that setup).

 

On the MacOS setup, as soon as I used a SDX2-36C tape, the capacity was

 

much better. I considered the possibility that the batch of SDX2-50C

 

tapes that I purchased were all bad, but I cannot understand how they

 

would all be bad in the same way as to fill at the same capacity. I may

 

try a further test of the SDX2-50C on a unix OS and see what sort of

 

capacity I get, but it still seems a strange occurence for a box of tapes

 

to all behave in the same manner.

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