On Wed, Jan 07, 2004 at 11:47:56PM +0100, Tilmann Haug wrote:
> Hi Amanda Experts,
>
> I´am new to amanda and get closer to a decend backup solution every day.
> Status: Amanda is up and running and even amrestore works fine. :-)
> I have an HP C1554A DDS3 Tapedrive in my Linux Box.
>
> The device has some jumpers on the underside. The two switches for
> hardware compression are both set to yes. According to the manual this
> means "hardware compression - on" and "host control - yes".
Clarification,
"hardware compression" on these drives from HP means "when powered on,
what will be the state of hardware compression". In your case "on".
The second switch controls whether this initial setting can be varied
by software commands from the computer. In your case "yes".
Also, note they only affect the drive when writing, not reading.
Reading senses the state data on the tape and switches automatically.
Unless you have a very specific and single use for the drive, I suggest
leaving the second switch set to "yes". The initial state switch setting
is your choice depending on which you expect to use most often. But with
the second switch set to "yes", you always must make certain your software
sets the HW compression to the state it wants.
> So my first question is: can Amanda use with these switch settings
> already hardware compression? How can i find out?
I do.
>
> Second question is about the DDS3 tapes.
Note, drives can "write" to tape a certain amount of data often called
the "native capacity". This is the amount of data the tape can hold, period!
If you can somehow squeeze more of your data into those same number of bytes,
then you can put more of your data on to the tape. But the tape still only
holds the "native capacity" number of bytes.
Amanda, if you plan on using software compression (gzip), as do many many
amanda users, wants to know the "native capacity" of your tape. HW vendors
often specify two numbers, the approximate native capacity and a higher
number their GUESS at how much squeezed data can fit into the native capacity.
HW vendors are optimistic (lie through their teeth?) about the larger number
and generally slightly high on the native capacity. Your "12GB" format will
probably hold 11.5 to 11.9GB.
> # We use an HP-DAT24 DDS-3 drive here (12GB native capacity),
> # here is the tapetype I use for it:
>
> define tapetype HP-DAT24 {
> comment "HP SureStore DAT24"# disabled compression
> length 12288 mbytes # 12 GB
> filemark 0 kbytes # Yes the filemark parameter is
> correct, # they're integrated into the data on
> speed 1024 kbytes # DDS-3's.
> }
Seems high for the capacity (length). Someone just calculated 12 * 1024.
Filemark is space between tape files that contains no user data. This can
be quite large for some tape formats. Not so for DDS3. Even if it were
500K, that is so tiny, 0.004% per tape file, that it does not matter.
Speed is for your info only, it is not used by amanda.
These data can be measured on your own system for your own drive with
the amanda program amtapetype.
--
Jon H. LaBadie jon AT jgcomp DOT com
JG Computing
4455 Province Line Road (609) 252-0159
Princeton, NJ 08540-4322 (609) 683-7220 (fax)
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