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Erik P. Olsen wrote:
> On 15/08/08 23:25, Martin Simmons wrote:
>>>>>>> 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.
>
> That I have but I don't know how to check if it works.
>
>>> 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.
>
> It also depends how the data is stored on the tape. I'll see if I can find
> out.
>
>> The other thing to note is that you might not know how full your tapes are
>> when using hardware compression.
>
> That I don't understand, AFAIK Bacula continues to write the tape till EOV and
> that should be the same with hardware, software, or no compression. Or have I
> misunderstood something?
What that means is you will not know how much space something will take
up on the tape. You can never figure out, as a result, what the size of
backed up data is going to be on the tape, so you don't REALLY know how
full it is. Even if you knew you had 10GB remaining, you don't know how
much data you can fit in that 10GB, as compression will change that
amount (even making files larger if the compression algorithm on the
tape drive is lousy).
- --
---- _ _ _ _ ___ _ _ _
|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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