{"id":287,"date":"2020-08-18T19:23:47","date_gmt":"2020-08-18T20:23:47","guid":{"rendered":"http:\/\/www.linux-tutorial.info\/?page_id=77"},"modified":"2020-08-22T19:26:39","modified_gmt":"2020-08-22T20:26:39","slug":"this-is-the-page-title-toplevel-122","status":"publish","type":"page","link":"http:\/\/www.linux-tutorial.info\/?page_id=287","title":{"rendered":"Tape Drives"},"content":{"rendered":"\nTape Drives<\/title>\n<p>\nFor the longest time, tape drives were a block to me. Although I understood\nthe basic concept (writing to a tape similar to a music cassette), it took me\nquite a bit of time before I felt comfortable with them.\n<\/p>\n<p>\nBecause this\ndevice has the potential for either saving your data or opening up career\nopportunities for you to flip burgers, knowing how to install and use them is an\nimportant part of your job as a system <glossary>administrator<\/glossary>.\n Because the tape device\nnode is usually read\/write, regular users can also back up their own data with\nit.\n<\/p>\n<p>\nThe first tape drives supported under Linux were quarter-inch\ncartridge tapes, or QIC tapes. QIC is not just an abbreviation for the size of\nthe media; it is also a standard.\n<\/p>\n<p>\nIn principle, a QIC tape is like a\nmusic cassette. Both consist of a long, two-layer tape. The “backing”\nis usually made of cellulose acetate (photographic film) or polyester (1970s\nleisure suits), polyester being more common today. The “coating” is\nthe actual media that holds the magnetic signals.\n<\/p>\n<p>\nThe difference is in\nthe way the tapes are moved from the supply reel to the take-up reel. In\ncassette tapes, movement is accomplished by a capstan and the tape is pinched\nbetween two rollers. QIC tapes spread the driving pressure out over a larger\narea by means of a drive belt. Additionally, more care is taken to ensure that\nthe coating touches only the read\/write heads. Another major difference is the\nsize. QIC tapes are much larger than cassette tapes (and a little bit smaller\nthan a VHS video tapes).\n<\/p>\n<p>\nInitially, the QIC tape was 300 feet long and\nheld approximately 30Mb of data. This was a DC300 tape. The tape that next\nappeared was a DC600, which was 600 feet long and could hold about 60Mb. As with\nother technologies, tape drives got better and longer and were able to hold more\ndata. The technology advanced to the point where the same tapes could be used in\nnew drives and could store as much as twice as much as they could before.\n<\/p>\n<p>\nThere are currently several different QIC standards for writing to tape\ndrives, depending on the tape and tape drive being used. Older, 60Mb drives used\na QIC-24 format when writing to 60Mb tapes. Newer drives use the QIC-525 format\nto write to several different kinds of tapes. As a result, different tapes yield\ndifferent capacity depending on the drive on which they are written.\n<\/p>\n<p>\nFor\nexample, I have an Archive 5150 tape drive that is “officially”\ndesigned to work with 150MB tapes (DC6150). However, I can get 120Mb from a\nDC600. Why? The DC600 is 600 feet long and the DC6150 is only 20 feet longer. A\ntape drive designed to use DC600 tapes only writes in 9 tracks, however, and a\ntape that uses DC6150s (like mine) writes in 15 tracks. In fact, you can use\nmany different combinations of tapes and drives.\n<\/p>\n<p>\nOne thing I would like\nto point out from a technical standpoint is that there is no difference between\n150Mb and 250Mb QIC drives. When the QIC standard was enhanced to include\n1000-foot tapes, 150Mb drives automatically became 250Mb drives. (I wish I had\nknown this before I bought so many DC6150 tapes. Oh, well, live and learn.)\n<\/p>\n<p>\nA similar thing happened with 320Mb and 525Mb tapes. The QIC-320 standard\nwas based on 600-foot tapes. However, the QIC committee decided to go with the\nQIC-525 standard based on 1000-foot tape. Thats why a 600-foot tape writing with\nthe QIC-525 standard writes 320Mb.\n<\/p>\n<p>\nNotice that this entire time, I never\nreferred to QIC–02 tapes. Thats because QIC-02 is not a tape standard, but a\ncontroller standard.\n<\/p>\n<p>\nAn interesting side note is just how the data is\nactually written to the tape. QIC tape drives use a system called\n“serpentine recording.” Like a serpent, it winds its way back and\nforth along the length of the tape. It starts at one end and writes until it\nreaches the other end. The tape drive then reverses direction and begins to\nwrite toward the other end.\n<\/p>\n<p>\nOther common tape drives are QIC–40 and\nQIC-80 tape drives, which provide 40Mb and 80Mb, respectively. These provide an\ninexpensive backup solution. These tape drives are connected to standard floppy\ncontrollers and, in most cases, the standard floppy cables can be used. The size\nof the tapes used for this kind of drive is about the same as a pack of\ncigarettes.\n<\/p>\n<p>\nAside from using the same type of controller, QIC-40\/80 tape\ndrives are similar to with floppy drives in other ways as well. Both use\n<em>modified frequency modulation <\/em>(MFM) when writing to the device. Sectors\nare assigned in similar fashion and each tape has the equivalent of a file\nallocation table to keep track of where each file is on the media.\n<\/p>\n<p>\nQIC-40\/80 tapes must be formatted before they are used, just like\nfloppies. Because the size of data storage is substantially greater than for a\nfloppy, formatting takes substantially longer. Depending on the speed of the\ntape drive, formatting can take up to an hour. Pre-formatted tapes are also\navailable and, like their floppy counterparts, the prices are only slightly\nhigher than unformatted tapes.\n<\/p>\n<p>\nBecause these tape drives run off the\nfloppy controller, it is often a choice between a second floppy drive and a tape\ndrive. The deciding factor is the floppy controller. Normally, floppy\ncontrollers can only handle two drives, so this is usually the limit.\n<\/p>\n<p>\nHowever, this limit can be circumvented if the tape drive supports\n<em>soft select<\/em> (sometimes called “phantom select”), whereby the\nsoftware chooses the device number for the tape drive when it is using it. The\nability to soft select depends on the drive. Though more floppy tape drives\nsupport this capability, many of the older drives do not. I will get into more\ndetail about this in the second part of the book when I talk about installing\nand using tape drives.\n<\/p>\n<p>\nOn larger systems, neither QIC nor mini-tapes can\nreally handle the volume of data being stored. While some QIC tapes can store up\nto 1.3Gb, they cannot compare to digital audio tape (DAT) devices. Such devices\nuse Digital Data Storage (DDS) media. Rather than storing signals similar (or\nanalogous) to those coming across the <glossary>bus<\/glossary>,\n DDS stores the data as a series of\nnumbers or digits on the tape, hence, the name “digital.” The result\nis much higher reliability.\n<\/p>\n<p>\nPhysically, DATs are the smallest tapes that\nLinux supports. The actual media is 4mm, so DATs are sometimes referred to as\n4mm tapes.\n<\/p>\n<p>\nHewlett-Packard DATs can be divided into multiple logical\ntapes. This is useful when making backups if you want to store different file\nsystems to different “tapes” and you don’t want to use any extra\nphysical tapes. Device nodes are created to represent these different logical\ntapes. <glossary>DAT<\/glossary>\ndrives can quickly scan for the location of subsequent partitions (as\nthey are called), making searches much faster than with backups to single tapes.\n<\/p>\n<p>\nOne thing to watch out for is that data written to DATs are not as\nstandardized as data written to QIC tapes. Therefore, it is possible that data\nwritten on one <glossary>DAT<\/glossary>\ndrive cannot be read on another.\n<\/p>\n<p>\nThere are two reasons\nfor this problem. This first is the <glossary>blocking<\/glossary>\nfactor, which is the minimum space\neach file will take up. A 1KiB file with a <glossary>blocking<\/glossary>\nfactor of 20 will have 19KiB\nof wasted space. Such a situation is faster in that the tape drive is streaming\nmore, though there is a lot of wasted space. <glossary>DAT<\/glossary>\ndrives use either a <glossary>variable<\/glossary>\nor\nfixed block size. Each drive has a default <glossary>blocking<\/glossary>\nfactor that is determined by\nthe drive itself.\n<\/p>\n<p>\nAnother problem is data compression, which, if it is\ndone, is performed at the hardware level. Because there is no standard for data\ncompression, it is very unlikely that two drives from different manufactures\nthat both do data compression will be able to read each others tapes.\n<\/p>\n<p>\nKeep in mind that that’s not all. There are many more standards that I\ndidn’t list here. One place to start is the QIC consortium’s home page at\n<i>www.qic.org<\/i>, which lists dozens of tape standards and associated\ndocuments.\n<\/p>\n<p>\nBefore you buy a tape drive, be sure to find out how easy it is to get the tapes\nand how expensive they are. I bought a tape drive once that was fairly\ninexpensive, but the tapes were hard to find and more expensive than others.\nEventually, I had to special order them from a distributor on the other side of\nthe country, because my local vendor stopped carrying them (I was the only one\nwho used them). The initial cost might have been more for a different tape drive, but\nI would have saved in the long run.\n<p>\n<b>Tape Loaders<\/b>\n<p>\nIf you have a lot of data to backup, tape loaders can be a real time saver.\nIn essence, a tape loader is a single tape drive with the ability to store\nmultiple tapes. Because the mechanism can load any tape you choose, they\nfunction similarly to music jukeboxes. As a result, tape loaders are sometimes\ncalled tape jukeboxes.\n<p>\nMost of the tape loaders I have seen come with either five or seven slots.\nYou can fill up all of the slots on Monday and write to a different tape each\nday of the week. Although this saves time, I would still recommend taking the\ntape out every day and storing it separately from the machines.\n<p>\nEven so, I still feel it is a time saver to fill the loader once on Monday\nfor the week, particularly if you have a large pool of tapes. For example, in\none company, we had enough tapes for a couple of months worth of backups. Our\nbackup software keep track of which tapes were in the drive as well as on which\ntape any given file resided. We checked once on Monday to see what tapes were\nneeded for the week, filled up the loader and then simply removed each tape as\nit was used.\n<p>\nOn Friday, we did a full backup of every file on the system. This required the\nloader be filled up completely, since we had some much data. Therefore, having the\nloader was a necessity for the weekend backups. Therefore, we simply used the\navailable functionality during the week.\n<p>\nAs the company and quantity of data grew, we eventually needed more tapes\nthan could fit in a single loader. That meant we had to get a second loader for\nthat machine. Although most of the more advanced backup packages can handle\nloaders not all of them work well with multiple loaders. Therefore, you should\ncheck in advance before buying something that cannot grow with you.\n","protected":false},"excerpt":{"rendered":"<p>Tape Drives For the longest time, tape drives were a block to me. Although I understood the basic concept (writing to a tape similar to a music cassette), it took me quite a bit of time before I felt comfortable … <a href=\"http:\/\/www.linux-tutorial.info\/?page_id=287\">Continue reading <span class=\"meta-nav\">→<\/span><\/a><\/p>\n","protected":false},"author":1,"featured_media":0,"parent":0,"menu_order":0,"comment_status":"closed","ping_status":"closed","template":"","meta":{"footnotes":""},"class_list":["post-287","page","type-page","status-publish","hentry"],"_links":{"self":[{"href":"http:\/\/www.linux-tutorial.info\/index.php?rest_route=\/wp\/v2\/pages\/287","targetHints":{"allow":["GET"]}}],"collection":[{"href":"http:\/\/www.linux-tutorial.info\/index.php?rest_route=\/wp\/v2\/pages"}],"about":[{"href":"http:\/\/www.linux-tutorial.info\/index.php?rest_route=\/wp\/v2\/types\/page"}],"author":[{"embeddable":true,"href":"http:\/\/www.linux-tutorial.info\/index.php?rest_route=\/wp\/v2\/users\/1"}],"replies":[{"embeddable":true,"href":"http:\/\/www.linux-tutorial.info\/index.php?rest_route=%2Fwp%2Fv2%2Fcomments&post=287"}],"version-history":[{"count":1,"href":"http:\/\/www.linux-tutorial.info\/index.php?rest_route=\/wp\/v2\/pages\/287\/revisions"}],"predecessor-version":[{"id":724,"href":"http:\/\/www.linux-tutorial.info\/index.php?rest_route=\/wp\/v2\/pages\/287\/revisions\/724"}],"wp:attachment":[{"href":"http:\/\/www.linux-tutorial.info\/index.php?rest_route=%2Fwp%2Fv2%2Fmedia&parent=287"}],"curies":[{"name":"wp","href":"https:\/\/api.w.org\/{rel}","templated":true}]}}