Re: [Bacula-users] Backup of many many files
2008-09-04 10:00:15
Hi,
04.09.2008 14:00, T. Horsnell wrote:
> FMI, will someone please tell me how shoeshining manifests itself?
> Does it mean that the tape actually reverses periodically?
Yes.
> e.g. to backspace over a filemark in order to overwrite it, or does it
> mean that the tape just stop-starts rather than continuously streaming.
> If its just stop-start, why does it result in more tape wear (as opposed
> to tapedrive wear) than streaming?
Because a tape can not be stopped and started in an instant - the tape
has mass, it moves with high speed, so even if you had a drive
mechanism that would be capable of starting and stopping
(near-)instantaneously, the tape would break.
What happens is this:
- Tape is driven at writing speed.
- Drive runs out of data and, after the last byte in its buffer is
written, it stops the tape. Some tape passes until the tape comes to rest.
- Drive gets more data
- Drive rewinds the tape until safely before the current end-of-data
on tape
- Drive starts moving the tape forward
- Once read speed is reached, the drive syncronizes to the tape tracks
and reads until the current end-of-data is reached.
- In this instant it switches to write mode and writes the current data.
Sometimes the first attempt to find the writing position will not
work, so this process can iterate a few times.
The result is that the tape passes the read/write heads a - sometimes
huge - number of times. Each pass rubs some matter from the tape and
the heads.
Furthermore, the strain on the tape itself is highest when accelerated
and stopped.
Todays tape technology, which writes hundreds of MB in seconds, really
suffers if you feed bursts of data to it - a GB of data, fed in small
bursts, can result in dozens of start-stop-cycles.
Another effect is that, when the write position is found, a small
length of tape can pass unused before actual data is written again, so
you lose tape capacity. Similarly, a file mark on tape can use a
considerable length of tape - both effects decrease the useable tape
capacity.
So you really want to avoid this shoeshining and lots of file marks.
Arno
> Cheers,
> Terry
>
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Arno Lehmann
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- Re: [Bacula-users] Backup of many many files, (continued)
- Re: [Bacula-users] Backup of many many files, Jesper Krogh
- Re: [Bacula-users] Backup of many many files, Chris Howells
- Re: [Bacula-users] Backup of many many files, Ryan Novosielski
- Re: [Bacula-users] Backup of many many files, Florian Heigl
- Re: [Bacula-users] Backup of many many files, Chris Howells
- Re: [Bacula-users] Backup of many many files, T. Horsnell
- Re: [Bacula-users] Backup of many many files,
Arno Lehmann <=
- Re: [Bacula-users] Backup of many many files, Ryan Novosielski
- [Bacula-users] Show Shining, was many many files, John Huttley
- Re: [Bacula-users] Show Shining, was many many files, Ryan Novosielski
- Re: [Bacula-users] Show Shining, was many many files, Jose Ildefonso Camargo Tolosa
- Re: [Bacula-users] Show Shining, was many many files, John Drescher
- Re: [Bacula-users] Show Shining, was many many files, Arno Lehmann
- Re: [Bacula-users] Show Shining, was many many files, T. Horsnell
- Re: [Bacula-users] Show Shining, was many many files, Arno Lehmann
- Re: [Bacula-users] Show Shining, was many many files, Dan Langille
- Re: [Bacula-users] Backup of many many files, Kjetil Torgrim Homme
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