On Tue, Aug 14, 2007 at 04:18:51AM +0200, 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
> }
>
LTO is unusual. When hwc is enabled each input block is compared
with and without compression. The smaller of the two is recorded.
As amtapetype feeds "random" data, data that is not compressible,
the original input block is taped. Thus you see the same results
with or without hwc enabled. And amtapetype can't tell whether
hwc is enabled or disabled.
...
>
> 2. The manual also mentions this:
> "When using hardware compression, change the length value based on the
> estimated compression rate. This typically means multiplying by
> something between 1.5 and 2.0"
>
> So, all I have to do is to run 'amtapetype' with HWC disabled and change
> "length 386048 mbytes" -> "length 579072" (something like that)
> afterwards. Is this true? Because at
>
> http://wiki.zmanda.com/index.php/Tapetype_definitions#LTO_Ultrium_3_with_400.2F800_Gbyte_tapes
>
> the examples for "HWC on" and "HWC off" are quite identical in length.
> So I am a little confused...
If some or all of your DLEs are not software compressed and you are
using hwc, there is no single value that is accurate every day. The
reason is that different data compress different amounts. Varying
widely, like not at all to 90 percent. As what you backup each
day varies, the degree of hwc will vary too.
So pick a value and watch the daily reports. See how much data is
actually written to your tapes. You will soon get a feel for whether
it is too low or high. If you never get a tape overflow then how
accurate your estimate is doesn't matter. If you regularly get
tape overflow the reports will tell you how much amanda was able
to write up to the end of tape. Compare several to approximate
the tapelength. Note, this will primarily affect amanda's estimate
and planning phase.
--
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)
|
