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Erik P. Olsen wrote:
> With tapes you have the option of using software or hardware compression or
> none
> at all. I have been using software compression several years expecting that
> this
> was the wisest to use.
>
> I have now been through a period of rebuilding my system due to a disk
> breakdown
> and that has meant a lot restores of mainly small amount of data. These
> restores
> have all taken a very long time to complete from when the tapes were mounted
> to
> the actual data was restored. I can see that bacula knows from the catalog in
> which tape file the data may be found. I assume it's very fast to go to the
> target file using forward space file, but what goes on when it then locates
> the
> data file within the tape file? I believe it's done by a sequential search of
> the tape file. I can see from the tray monitor that it reads block after block
> before the restore takes place. This seems to cost a lot of time especially if
> the blocks have to be unzipped. If this is true a better restore performance
> will be obtained with uncompressed tapes or even with hardware compressed
> tapes
> because here the data is decompressed in flight.
>
> If all this is true I tend to believe that hardware compression is by far the
> best method. On the other hand I know that software compression is advocated
> by
> folks more knowledgeable than me.
>
> Would someone please explain what's up and down with this sort of performance?
A quick answer would be that it depends on the type of device. I've
heard some new tapes have very good hardware compression that eliminate
some of the problems one might have with older methods (performance,
lousy compression, making compressed files larger, etc.) whereas some
older technologies don't. You could probably look at a review of
hardware compression across different types of tape drives to see what's up.
- --
---- _ _ _ _ ___ _ _ _
|Y#| | | |\/| | \ |\ | | |Ryan Novosielski - Systems Programmer II
|$&| |__| | | |__/ | \| _| |novosirj AT umdnj DOT edu - 973/972.0922 (2-0922)
\__/ Univ. of Med. and Dent.|IST/AST - NJMS Medical Science Bldg - C630
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