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DAT160 - negative compression ratio?

 
Flyin'High
Occasional Advisor

DAT160 - negative compression ratio?

Dear HP,
i am really confused. I have backed-up my scan archive to tape (~130 Gb subtotal) and soon noticed, that only 72 Gb fits on a 160* media. At the same time, Data Protector reports 78 Gb of native capacity.

After a short exploration in depth found that a 0.9:1 ratio is displayed for 2nd media part.
SW compression was turned off, in fact.

My question is divided into two major subjects:
1) It's really so? Data is 48 bit TIFF (uncompressed), all file sizes between 100 and 400 Mb, generally.

Another tape - JPEG & RAR - shows even 0.8:1
Device firmware is WP74

2) Do you have any "estimation" table (or a s/w tool) for different data types - DV AVI, for example, large (gigabyte) WAVEs, et.c.
DCLZ paper doesn't help

How is best to know when hardware compression needs to be turned off, and when it's not? In fact, should be rather tricky on incremental (raster & vector) backups...

Regards,
4 REPLIES 4
thomasr
Respected Contributor

Re: DAT160 - negative compression ratio?

Already compressed data such as jpeg, mpeg, and zip files may exhibit behaviour like you're seeing. If data has been compressed well, it normally won't compress again, and there is an overhead in the metadata that the second compression attempt will put around the data.

HP's Library and Tape Tools ( www.hp.com/go/tape ) under the "Sys Perf" button, will do a read of a filesystem and report on compressibility.

--
Liberty breeds responsibility; Government breeds dependence
Flyin'High
Occasional Advisor

Re: DAT160 - negative compression ratio?

Thank you. I have benchmarked the filesystem (1.08 gain maximum on DV).

But passing large TIF files is very slow. Really slower, than a small ones to be honest...

Compare - 220 Mb/s falls to 116 on a 72 media.
(and Is a throughput an indirect mark of a file compression ratio?)
Flyin'High
Occasional Advisor

Re: DAT160 - negative compression ratio?

Of course, text above should be read as "Mb/min". Sorry my typo :(
Highlighted
thomasr
Respected Contributor

Re: DAT160 - negative compression ratio?

Try using your backup application to turn HW compression off when you know that a significant portion of your data is already compressed. This is more of an issue for DAT and DLT drives -- I believe that LTO drives have a split pipeline, where data is sent to compression engine and non-compression simultaneously; if compression is < 1:1, it uses the uncompressed data.

Large files do normally have good performance -- it's when you're backing up small files (less than 100K are slower; less than 10K can be painful) that performance starts to suffer.

--
Liberty breeds responsibility; Government breeds dependence