Re: Odd error from amcheck

> On Apr 17, 2019, at 10:10 PM, Jon LaBadie <[email protected]> wrote:
> 
> On Wed, Apr 17, 2019 at 11:00:11PM +0000, Debra S Baddorf wrote:
>> 
>>> On Apr 17, 2019, at 5:40 PM, Jon LaBadie <[email protected]> wrote:
>>> 
>>> On Wed, Apr 17, 2019 at 06:24:42PM +0000, Debra S Baddorf wrote:
>>>> 
>>>>> On Apr 17, 2019, at 12:15 PM, Jason L Tibbitts III <[email protected]> wrote:
>>>>> 
>>>>> Has anyone seen anything like this?  What's the magic incarnation to
>>>>> read the amanda label from a tape without running amanda tools?  (I know
>>>>> dd should do it but I can't seem to find the right combination of
>>>>> options.)
>>>>> 
>>>>> - J<
>>>> 
>>>> 
>>>> mt -f /dev/nst0   rewind
>>>> dd if=/dev/nst0   of=MyFile   bs=512k   
>>>> mt -f /dev/nst0   rewind     !! so ready for amanda to use it
>>>> 
>>>> for   /dev/nst0   substitute the location of your drive   (grep amanda.conf  to find it)
>>>> 
>>>> for bs=512k   you may have to guess higher or lower (if an older, smaller tape).
>>>> This is usually the hardest part to guess at.
>>>> 
>>>> 
>>>> Then do     more MyFile
>>>> read the first block   and ^Q   out of the rest.   
>>>> 
>>>> Mine shows    
>>>> AMANDA: TAPESTART DATE 20190204200001 TAPE ad5-daily-123
>>>> ^L
>>>> 
>>> 
>>> Isn't the amanda tape header data still 32KB?  I thought you
>>> could skip syncronizing dd block size and actual tape bs.
>>> 
>>> For example:
>>> 
>>> # dd if=/dev/???? of=MyFile bs=32k count=1
>>> 
>>> Skip the "of=MyFile" to write to stdout/screen.
>>> 
>>> I think you would have to synchronize the bs of dd and the
>>> tape if you wanted to read file data from the tape.  But
>>> not the 1st 32KB.
>>> 
>> 
>> Can it do mixed block sizes on the same tape?   Cuz I know the rest of my blocks are 512k,
>> possibly by my choice.   For LTO5 tapes.    (I had read that larger tapes need a larger block size.)
>> Oh here it is:   
>> grep  BLOCK_SIZE   amanda.conf
>>      device-property "BLOCK_SIZE"  "512k"   # from man amanda-changers.7
>> to see what block size you are using.   If you don’t find the term,  then perhaps it’s a default 32k
>> The “from man amanda-changers.7”  was a note to myself.
>> 
>> 
>> When I omit  the blocksize,  I get  “Cannot allocate memory”:
>>     dd: reading `/dev/nst0': Cannot allocate memory
>> which isn’t an obvious error, but always goes away when I find the right blocksize to specify.
>> 
>> It fails to read ANY blocks, until I get the right blocksize.    I’m only looking
>> at the tape-label   first file.   I think the tape-label uses the block size you’ve specified, for the whole tape.
>> 
>> dd if=/dev/nst0  bs=512k    works fine  (straight to standard out)
>> dd if=/dev/nst0                    fails, and gets the above error.   For me.
>> 
> 
> Sorry Deb, you and Jason have demonstrated my lack of experience
> with large tapes.
> 
> I did assume the tape header was written in the same size block
> as the rest of the tape, 512KB in this case.  But I expected
> that "dd" could read a partial block.
> 
> I now have a dim memory that dd must have a block size at least
> as big as the device it is reading from.  Otherwise dd's input
> buffer overflows.
> 
> I was disappointed to see that dd's "count" option is based on
> input blocks.  But I found a flag that lets me output only 32K
> with a large block size.  At least with regular files as input.
> I don't have large tapes to test.
> 
>  dd bs=2M if=<your_device> iflag=count_bytes count=32K
> 
> I stuck a 2M block size in there thinking it would be a nice
> multiple of any real block size.
> 
> The iflag=count_bytes is the flag that converts count=###
> into bytes rather than blocks.
> 
> Sorry again for the fuzz with my earlier post.
> 
> Jon
> -- 
> Jon H. LaBadie                 [email protected]
> 11226 South Shore Rd.          (703) 787-0688 (H)
> Reston, VA  20190              (703) 935-6720 (C)


No sweat!   Both of you have taught me something new:
I can tell it any block size, as long as it is bigger than my tape actually uses.
THAT may be very useful!!   No more futzing to guess what is being used.
Just pick a really big number!

Deb Baddorf