Astrophysics Research CentreQueen's University Belfast • School of Mathematics & Physics |
Tape DrivesA number of tape drives are located on jadzia (coffee room). You can use any of these drives simply by sshing into the machine - your home and scratch areas will appear as usual. We have four types of tape drive on jadzia. Exabyte tapes are no longer supported. Drive types
Using the drives
There are no particular special methods needed to use any of the drives, they use the standard tar, dump, and dd commands. However, on the DAT drive there may be a problem with the default method Linux uses for handling of the block sizes used for reading and writing. The practical effect of this is that a tape written on that drive may be read on it, but most likely not on any other machine. I have configured the systems to work around this issue, but it's best to be sure no one has altered the setup before writing to a tape. After inserting a DAT, type the following command:
mt -f /dev/dat status The output may look like this:[rsir@jadzia ~]# mt -f /dev/dat status mt -f /dev/dat setblk 0 which will make the drive use variable length blocks. The tape block size will then show up as 0 when the status command is run. This allows the drive to read tapes written on other machines, and to write tapes that may be read on other systems too.A note on compression
All of the tape drives in the division will apply hardware-based compression to the data written to tape, permitting you to store more data
on a tape. However, there are some aspects to this that may not be immediately clear. For example, when a DDS4 DAT claims a capacity of 20/40 GB, this means that the tape can definately hold 20GB of data, and may, assuming that all the data compresses with a 2:1 ratio, hold 40GB. The exact level of compression depends on the data being stored - text files tend to compress very well, so may give more than 2:1 compression. Images, in contrast, may not compress well at all. Data that is already compressed - e.g. MP3 files, JPEGs, gzipped giles, PDFs, will not compress significantly, and may even be made larger on tape by the hardware compression routine, though most modern drives are smart enough to write the uncompressed data in this scenario.
The basic principle to understand is that the maximum rated capacity quoted on any tape is somewhat idealistic (AKA marketing BS!), and not guaranteed. If you need to fit a large amount of data on a single tape, it may be more reliable to gzip/bzip the raw data first, and then write this out to tape - this way you will know for sure how much data you need to fit on the tape before you start. In addition, since the
software compression works on the whole of the data to be stored it can be much more efficient than the 'on the fly' hardware system, which necessarily only operates on small chunks of data at a time.
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