Tape Capacity FAQ
1. Why does the amount I can store on my tapes vary?
The majority of tape drives these days are streaming (e.g. DAT, 8mm, AIT, LTO, DLT, VXA). With streaming tape drives, the tape drive formats the tape on the fly, as it writes data, and the amount of data you can save on any tape depends on many factors, including how many times the tape drive has to start and stop while writing, as well as the condition of the tape (some tape drives perform read-after-write verification, and re-write blocks of data multiple times, if the read-after-write verification fails on some blocks). Because these variables are almost never the same for different backup sessions, the amount of data you manage to store on any particular tape will vary with streaming tape drives.
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.
Source Speed: 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 local backup 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.
The size of files being copied has a dramatic influence on the source speed. Tens of thousands of small files take longer to back up than a few very large files.
Compression: Many streaming tape drives contain a chip to compress the data coming into the drive before it is written to tape, improving speed and capacity. 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. If you are backing up pre-compressed data like videos, photos or audio files, you may want to disable hardware compression to increase your tape capacity.
Tape Length: Many tape drives support different length tapes. Obviously the longer the tape, the higher the capacity. Refer to the documents that came with your tape drive for more information on supported tape formats and basic capacities for each.
Media Failures: If a tape fills up sooner than expected, you may have encountered a media failure, or error 206. Your first step should be to look in the operations log to see if this or another error is responsible for Retrospect asking for a new piece of media. For more information regarding media failures, look up error 206 on our website.
2. My tape drive vendor promised me more capacity per tape than I am getting in my actual backups. What is going on?
Tape drive and tape media vendors tend to assume that using compression will give you a 50% reduction in the space used by your data. While with some data sets this is possible, the average user only sees about a 30% or less space savings when using compression.
Even when you are not using compression, software- or hardware-based, you may get lower capacity than the tape drive manufacturer advertised, depending upon your environment and usage dynamics. As mentioned above, with streaming tape drives, each time the tape drive stops to wait for more data, the capacity of the tape is slightly reduced. While performance with different tape drives varies dramatically, you will tend to get lower capacity on tapes used for network backups than for tapes used for backups of local fast hard disks. This lower capacity does not indicate a problem - it is simply how this tape drives work.
Ultimately, you can and should review the capacity that you regularly get on your tapes (by viewing the Members tab of the backup set Properties window (Windows) or Media Set Members tab (Macintosh). You will very quickly come to learn what to expect in your computing environment.
3. How can I tell how much more space is left on a tape?
With streaming tape drives the available space on a tape depends on many factors. A good place to start is Retrospect’s estimate of the available space. If, however, over time you find that Retrospect consistently over or under estimates the total space available for you to use on your tapes, based on the data you tend to back up, you can override the numbers Retrospect uses manually (Windows only). You do not need to do this, though, as Retrospect will always continue to write to a tape until the tape drive tells Retrospect that the tape is full.
To see Retrospect’s estimated free space, Windows users can go to the Configure tab in the Retrospect Directory and click Backup Sets. Highlight the backup set you want to view, and click the Properties toolbar. Macintosh users go to Media Sets>Members. Note in the Summary tab how much space Retrospect reports as available.
4. What kind of compression does Retrospect use?
Retrospect optionally uses either software compression or hardware compression, depending upon the device you are backing up to, and the options you set in Retrospect.
Hardware compression: Currently the only devices Retrospect supports for backup that use hardware compression are tape drives. You choose to enable or disable hardware compression on a set by set basis, when you create the backup set (there is a checkbox, default on, that tells Retrospect to use hardware compression if it is available). If your device does not support hardware compression, leaving the checkbox set does no harm. If you choose to encrypt your backup set, Retrospect disables hardware compression, as encrypted data does not compress well.
Software compression: You can tell Retrospect to compress data while backing up to any device using Retrospect’s software data compression. This may make the backup take a bit longer, but typically saves substantial space. If you are using a backup device that supports hardware compression, Retrospect lets the device compress the data, as this is faster. If you enable compression and encryption in Retrospect, Retrospect first compresses the data, and then encrypts it.
Retrospect compresses files into its own proprietary format using a "lossless" compression process. This means that Retrospect compresses your data in such a way that all original bytes of your source data are preserved. This is similar in concept to the popular .zip compression which compress and decompress preserving the original data byte for byte. If you back up a file using Retrospect’s software data compression, and then you restore that file, you get back every byte, exactly as it was when you performed the backup.
In contrast, some compression methods such as MPEG, compress files using a "lossy" compression algorithm, which actually discards some content in favor of a smaller file. For obvious reasons, lossy algorithms aren’t appropriate for backup applications, but work fine for many audio or video applications. Retrospect does not use these compression methods.
5. What is the difference between hardware compression and software compression?
Hardware compression is handled by the tape device, and is usually faster than software compression. When using hardware compression, Retrospect sends uncompressed data to the drive and lets the tape drive compresses the data before writing it onto the tape. Software compression is handled by Retrospect, which compresses blocks of data before sending it to the backup device.
If you are backing up to a tape drive that supports hardware compression, your backup set was created with hardware compression enabled, and you also enable software compression in Retrospect, Retrospect silently disables its software compression, because hardware compression is generally faster.
6. How can I tell if I am using hardware compression?
By default, Retrospect enables hardware compression in all tape backup sets. If you want to create a backup set that does not use hardware compression, simply create a new backup set, unchecking the hardware compression options before saving the catalog. You can only specify whether or not to use hardware compression when you create a backup set.
Not all tape drives support hardware compression. The best way to find out if yours does is to review the materials you received from the tape drive manufacturer, or to contact them directly. There is no need to disable hardware compression if your drive does not support it. All LTO devices come with hardware compression.
If your drive supports hardware compression, you can verify that your backup set has hardware compression enabled by viewing the backup set properties>Summary (Windows) or by going to Media Sets on the Macintosh.
7. Can I turn on hardware compression after I have already backed up to a backup set?
No, you can only enable or disable hardware compression when you first create a backup set. If you want to switch modes for a tape, you need to erase the tape, forget the backup set from within Retrospect, throw away your old backup set catalog, and create a new backup set with the option set the way you want it.
8. Can I use hardware compression with encryption?
No. Because encrypted data does not compress well, Retrospect does not allow you to use hardware compression with encrypted backup sets (the option to use hardware compression is automatically disabled if you choose to encrypt a backup set). You can, however, enable Retrospect’s software compression if you are using encryption. When you do this, Retrospect compresses the data before encrypting it, which gives you both security and reduced data size.
9. My tape drive doesn’t support hardware compression, but my tapes advertise a native and compressed capacity. What does that mean?
If your tapes come with materials that advertise compressed and uncompressed capacities, this simply means that these tapes can be used in drives that support hardware compression, as well as those that do not. If your drive does not support hardware compression, use the lower of the two figures in your planning.
If you have a tape drive that does not support hardware compression, you can optionally enable software compression, which will allow you to store more on each tape than if you back up the data uncompressed. Enable software compression in the Options window for your script or immediate operation. Click on the word "Options" or on the icon next to it in any summary window, and then enable or disable the Data Compression checkbox, as you wish.
10. What is the point of the "Set Capacity" button on Windows?
In a backup set Properties window (Windows, there is an option to override Retrospect’s default calculation of how much space is remaining free on a tape. Retrospect has a default value for all tape formats that it supports, but if you find that you consistently get more or less on your tapes, you can override the value used by Retrospect. The Macintosh version does not offer this feature. There is no need to do this, because Retrospect will continue to use a tape until the tape is filled. In other words, Set Capacity button is only used to help Retrospect display a more accurate remaining space free figure, not to actually control how much data will be copied. The Macintosh version will automatically adjust these estimates. In the end, Retrospect is going to always write data until the hardware reports the tape is full. Once a tape fills, a new tape will be requested.
11. Will the Set Capacity button (Windows only) affect how much data I can put on a tape?
No. Retrospect uses either its built-in default capacity value, or the one you enter manually via the Capacity/Set Capacity button for display purposes only. You never need to adjust this value, but you may if you consistently find that the capacity you get on your tapes is significantly higher or lower than Retrospect’s default value. Regardless of the capacity value used, Retrospect continues to write to a tape until the tape drive tells Retrospect that the tape is full.
12. How do I get to the Set Capacity button on Windows?
Go to the Configure tab in the Retrospect Directory and click Backup Sets. Highlight the backup set you want to view, and click the Properties toolbar button (Windows). Click the options tab, and then click the Capacity/Set Capacity button.
Last Update: 19 août 2014