>>>>> On Fri, 15 Aug 2008 12:14:09 +0200, Erik P Olsen said:
> 
> 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.

I don't think this makes any difference.  The software decompression is
handled by the file daemon, so it works on the backup data, not the blocks.
There should be no decompression until it reaches the part of the tape file is
being restored.

Check that you have Fast Forward Space File = yes in your SD config and that
it works for your drive.


> 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?

I agree with Ryan -- it depends on your tape drive technology.

The other thing to note is that you might not know how full your tapes are
when using hardware compression.

__Martin

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