Re: HP-DAT72 USB definition

On 2006-02-22 11:21, Foro Linux wrote:

After about 12 hours of amtapetype execution, here it is:

define tapetype HP-DAT72 {
comment "HP-DAT72 USB interface (compression on) (tapes HPreference: C8010A)"
    length 31267 mbytes
    filemark 524 kbytes
    speed 1474 kps
}



Does it make sense to use the length value of a tape drive having
hardware compression on?

If you want to use software compression, then you better disable
hardware compression, because applying both *expands* the data,
resulting a loss of effective tape capacity of about 15-20%.
In that case you better set it closer to 36 Gbyte (= 33.5 Gibyte)
giving you about 2 gigabyte extra capacity for such a tape.

If you want to use hardware compression, then add to the 36 Gbyte
some arbitrary percentage of what you hope the data will compress,
like some 30% compression, giving 33.5 GiB / 0.70 = 50 Gibyte.

Note that the prefixes in Amanda (K, M, G) are multiples of 1024,
and not 1000, in ISO notation that should have been Ki, Mi and Gi.
That is why a 36 Gbyte tape holds actually 33.5 Gibyte.
See: http://en.wikipedia.org/wiki/Binary_prefix .


--
Paul Bijnens, xplanation Technology Services        Tel  +32 16 397.511
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