On 2007-08-14 04:18, Ralf Auer wrote:

Hello everybody,

        if you don't mind, I have two questions concerning hardware compression.

I have two HP Ultrium 960 drives. Up to now I used them with hardware
compression disabled and compressed my data on the clients.

Now I enabled hardware compression and ran amtapetype.

1. The manual says (in Question 12):

"Reasons to run amtapetype for your device:...
- You want to determine if your device has hardware-compression enabled...

and some websites also claim that 'amtapetype' should print a warning
message when HWC is enabled. For some reason, it does NOT on my tape
server. It also reads "hardware compression off" in the final output:

define tapetype LTO3-HWC {
    comment "HP StorageWorks 960 LTO3 (hardware compression off)"
    length 386048 mbytes
    filemark 0 kbytes
    speed 65033 kps
}

I checked HWC several times with standard Unix- as well as with official
HP-software, so that I can say for sure that HWC is now enabled but not
recognized by 'amtapetype'. (I used Amanda 2.5.2p1 and 2.4.5p1)
Should I worry about that?

So the amount of data that amtapetype prints will be the same in both
cases (because amtapetype tests the capacity with uncompressable data).

But amtapetype should actually detect the hardware compression, unless
your tapedrive is too fast for the bus.
Amtapetype detects hardware compression by comparing the write speed
of very compressable data versus uncompressable data.
Because the write speed of tape is usually limited by the bits written
to the physical tape, amtapetype can measure the effect of compression
inside the tapedrive, by feeding lots of very compressible data, and
measuring the time it takes to write that to tape.
If the computer is too slow feeding the data to the tapedrive, this
process fails of course.  Because now the rate is limited by the bus,
instead of the physical tape writing, amtapetype cannot notice that
difference anymore.

You can run the compression test separately with the option "-c".
That takes only a few minutes at most.

I believe the native speed of an LTO-3 is 80 MB/sec?  And you
get 65 MB/sec, which is reasonably good.
So I don't understand why the compression-test does not work...


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