5 ms·

Basically, tape has a very very high latency (on the order of minutes to hours) for random access and it has a very low number of writes (usually a few dozen) b

by zaarn 7y ago

Basically, tape has a very very high latency (on the order of minutes to hours) for random access and it has a very low number of writes (usually a few dozen) before you need to copy it to a new medium. The drives are also quite expensive and if you use them for backups, you need to keep one ready in case the other borks it but you need the tape's data. Lastly you need specialized tools for access since most average tools will mishandle the tape.

peter_tonoli 7y ago
Not sure where you've got the 'few dozen' number from - that's not to my experience. Tandberg quote "1,000,000 passes on any area of tape, equates to over 20,000 end to end passes/260 full tape backup" http://www.tandbergdata.com/default/assets/File/Data_Sheets/DS_LTO_media_US.pdf http://www.tandbergdata.com/default/assets/File/Data_Sheets/...
- zaarn 7y ago
  Atleast from what I've been told and from experience, if you rotate over master backup tapes every few months, the tape will be almost 6 years old by the time it reaches the first two dozen end to end writes, that usually means newer LTO standards, newer drives and newer tapes on the market to replace them with.
- Dylan16807 7y ago
  > 1,000,000 passes on any area of tape, equates to over 20,000 end to end passes What in the world does that first number mean? My best guess is that it's some nonsense math based on the fact that the tape head contacts 32 tracks at once. So writing a full tape is 1 "full pass", which equals 208 "end to end passes" as the tape feeds back and forth, which equals 6656 """passes on any area""".
adrian_b 7y ago
My LTO-7 drive needs about 2 minutes from one end of the tape to the other, so that is the maximum latency when the tape is already loaded. Adding another minute to manually find and load a tape, would reach a 3 minute latency. So a few minutes of latency in the worst case, yes, but certainly not hours of latency, unless you must drive a car to an off-site location, but that would be the same for a HDD or optical disc stored in another place.
- zaat 7y ago
  Few hours of latency are expected when backups stretch across multiple tapes and recatalog of the tapes is required (the original backup server is dead). Having to do it manually with a single reader because your tape library is dead will slow the operation even more.
  - adrian_b 7y ago
    That is a problem of the backup software, not of the tapes. I have a custom program that generates an index with all the files on all my tapes and which updates the index after any tape write. I can determine in a second the location of any file, so I know immediately which tape cartridge must be loaded to retrieve anything. The file index is actually a relational database table with a lot of metadata about the files, including: position in the tape collection as tape number + file number, name, path name, modification time, length, content hash (to identify duplicates) and a set of keywords and tags, to be able to search for a file even when the name is not known, or to be able to retrieve sets of files. The files written on the tape are grouped in compressed archive files, but the database with metadata is about the files inside of those archive files. Obviously, the file index itself is also backed up regularly.
    - zaat 7y ago
      That's essentially what most backup software is doing. Restoring the index database from the tape and matching the physical tapes in the set to their logical names in the index gets complex and time consuming when you have many tapes in a set. It gets worse when you have to use more than one set because there is too much data to have full backup daily and you also need to index few incremental sets.
      - adrian_b 7y ago
        Restoring the index database from a tape would be needed only if the SSD where my database is normally located would become defective, which should be a very seldom event, especially if it is replicated on a few SSDs. In my custom program & file database, I only use physical tape names, i.e. the names that I write on the labels of the cartridges. Either the program tells me what cartridge to load in the drive, or I inform it what cartridge I have loaded (e.g. for a new cartridge). I see no reason for a logical name. If I would ever copy a tape on another and I would discard the original tape, it would be trivial to substitute the name in the database. Maybe for a large enterprise maintaining a comprehensive file database could be time consuming even in the best implementation, but I doubt it. In my experience, keeping an up-to-date file database for about 120 TB of content that increases by about 10 to 20 GB per day still takes a negligible time. While I have worked in a few large companies where a lot of data existed, in all small- and medium-sized companies (less than 100 computers) that I have seen there was less backup data than that (not counting things like standard disk images with the OS and with SW, which do not belong in periodic backups and which where shared, being identical for various classes of employees).

by zaarn 7y ago

peter_tonoli 7y ago

Not sure where you've got the 'few dozen' number from - that's not to my experience. Tandberg quote "1,000,000 passes on any area of tape, equates to over 20,000 end to end passes/260 full tape backup" http://www.tandbergdata.com/default/assets/File/Data_Sheets/DS_LTO_media_US.pdf http://www.tandbergdata.com/default/assets/File/Data_Sheets/...

zaarn 7y ago
Atleast from what I've been told and from experience, if you rotate over master backup tapes every few months, the tape will be almost 6 years old by the time it reaches the first two dozen end to end writes, that usually means newer LTO standards, newer drives and newer tapes on the market to replace them with.
Dylan16807 7y ago
> 1,000,000 passes on any area of tape, equates to over 20,000 end to end passes What in the world does that first number mean? My best guess is that it's some nonsense math based on the fact that the tape head contacts 32 tracks at once. So writing a full tape is 1 "full pass", which equals 208 "end to end passes" as the tape feeds back and forth, which equals 6656 """passes on any area""".

adrian_b 7y ago

My LTO-7 drive needs about 2 minutes from one end of the tape to the other, so that is the maximum latency when the tape is already loaded. Adding another minute to manually find and load a tape, would reach a 3 minute latency. So a few minutes of latency in the worst case, yes, but certainly not hours of latency, unless you must drive a car to an off-site location, but that would be the same for a HDD or optical disc stored in another place.

zaat 7y ago
Few hours of latency are expected when backups stretch across multiple tapes and recatalog of the tapes is required (the original backup server is dead). Having to do it manually with a single reader because your tape library is dead will slow the operation even more.
- adrian_b 7y ago
  That is a problem of the backup software, not of the tapes. I have a custom program that generates an index with all the files on all my tapes and which updates the index after any tape write. I can determine in a second the location of any file, so I know immediately which tape cartridge must be loaded to retrieve anything. The file index is actually a relational database table with a lot of metadata about the files, including: position in the tape collection as tape number + file number, name, path name, modification time, length, content hash (to identify duplicates) and a set of keywords and tags, to be able to search for a file even when the name is not known, or to be able to retrieve sets of files. The files written on the tape are grouped in compressed archive files, but the database with metadata is about the files inside of those archive files. Obviously, the file index itself is also backed up regularly.
  - zaat 7y ago
    That's essentially what most backup software is doing. Restoring the index database from the tape and matching the physical tapes in the set to their logical names in the index gets complex and time consuming when you have many tapes in a set. It gets worse when you have to use more than one set because there is too much data to have full backup daily and you also need to index few incremental sets.
    - adrian_b 7y ago
      Restoring the index database from a tape would be needed only if the SSD where my database is normally located would become defective, which should be a very seldom event, especially if it is replicated on a few SSDs. In my custom program & file database, I only use physical tape names, i.e. the names that I write on the labels of the cartridges. Either the program tells me what cartridge to load in the drive, or I inform it what cartridge I have loaded (e.g. for a new cartridge). I see no reason for a logical name. If I would ever copy a tape on another and I would discard the original tape, it would be trivial to substitute the name in the database. Maybe for a large enterprise maintaining a comprehensive file database could be time consuming even in the best implementation, but I doubt it. In my experience, keeping an up-to-date file database for about 120 TB of content that increases by about 10 to 20 GB per day still takes a negligible time. While I have worked in a few large companies where a lot of data existed, in all small- and medium-sized companies (less than 100 computers) that I have seen there was less backup data than that (not counting things like standard disk images with the OS and with SW, which do not belong in periodic backups and which where shared, being identical for various classes of employees).