On 2012-08-21 05:42, Alan Brown wrote:
> On 21/08/12 08:37, lst_hoe02 AT kwsoft DOT de wrote:
>> Some short tests with the "Maximum Block Size" set, show transfer
>> speed to the LTO-4 of 87MBytes/sec with 256K and 98MBytes/sec with 
>> 1M
>> with Bacula encrypted data, so with this we are reaching the
>> theoretical uncompressed speed of the LTO-4 device by ~20%. It would
>> be really helpful if there is some developer to say if there is any
>> downside of large block sizes?
>
> I'm not a dev, however I use 2Mb block size (this is the largest 
> bacula
> currently supports)

I suspect I'm an edge case with tape.  I'm using old technology and old
hardware.  That's what I can afford on a home network.

I wasn't sure what block size I'm using on my DLT-7000 tape library, 
but
I see it's variable....

$ sudo mt -f /dev/nsa0 status
Mode      Density              Blocksize      bpi      Compression
Current:  0x1b:DLTapeIV(35GB)    variable       85937    IDRC
---------available modes---------
0:        0x1b:DLTapeIV(35GB)    variable       85937    IDRC
1:        0x1b:DLTapeIV(35GB)    variable       85937    IDRC
2:        0x1b:DLTapeIV(35GB)    variable       85937    IDRC
3:        0x1b:DLTapeIV(35GB)    variable       85937    IDRC
---------------------------------
Current Driver State: at rest.
---------------------------------
File Number: 0  Record Number: 0        Residual Count 0

All my tape jobs involve data on local HDD.  That is, I backup to disk 
first, then
copy to tape.  The HDD and the tape are both connected to the same 
bacula-sd.

> The only downside is that you _must_ mark all active tapes as "used"
> before changing block size.

Setting them as USED is fine by me.  I write my tapes until they are 
FULL.
Checking the recycling algorithm documentation[1], this will not affect
pruning.

> Bacula will be able to read the old tapes, but it cannot handle 
> maximum
> block size changing midway through a tape.

I found one reference[2] after a bunch of searching, but I should look 
more:

###
The DLT drive default data block transfer size is 4KB (4096 bytes).
To achieve better performance, adjust block size to 32K bytes or
higher when using a fixed block device.
###

My full backups, roughly 90GB, take about 12 hours to copy to tape.  
Thus,
my only incentive to change blocksize is to avoid shoe-shine, which I 
do not
think is happening at present.


[1] - 
http://www.bacula.org/5.2.x-manuals/en/main/main/Automatic_Volume_Recycling.html

[2] - 
http://techpubs.sgi.com/library/dynaweb_docs/hdwr/SGI_Admin/books/DLT_OG/sgi_html/ch05.html

-- 
Dan Langille - http://langille.org/


-- 
Dan Langille - http://langille.org/

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