Re: mt and tar fail on LTO-5 drives

[Corinna Vinschen <corinna-cygwin at cygwin dot com>]

On Aug 17 20:35, bartels wrote:
> Hello People,
> 
> I have here two SAS connected LTO-5 drives: one IBM and one HP.
> 
> Both drives work work fine, but sadly mt does not.
> The size reported by mt is a meager 35 GB, instead of the expected 1.5TB
> 
> I have tried both an older 32 bit and the 'current' 64 bit cygwin: same result.
> 
> Writing to tape works fine with tar, but the tape is quickly considered 'full'.
> Is there any hope of fixing this? LTO-6 is already out there.

I don't know.  Cygwin uses the Win32 tape API.  The OS function
GetTapeParameters returns the capacity and the # of remaining bytes as 8
bytes LARGE_INTEGER value.  The size of LT)-5 or LTO-6 should fit easily
into that.

I just checked that the value is stored within Cygwin as 8 byte long
long value, so no problem there.  The mt tool prints the value as %lld
value, so it should print it correctly as 8 byte value.  From what I can
see, the wrong value *seems* to be returned by the OS.

Also, the write(2) function does not check for the remaining bytes, so I
wonder why tar should fail prematurely, unless there's a problem with
the block number.  The OS function GetTapePosition returns the current
block number as LARGE_INTEGER, but Cywin stores it in a 32 bit int.  So
the block number overflows after 4 billion blocks.  But even with a
small blocksize of 512 bytes this would only occur at about 2 TB of
data, long after the end of the tape.  Despite that this is more of a
theoretical problem, the mtop struct to pass parameters to ioctl(fd,
MTIOCTOP, ...) only allows 4 byte count values anyway, even on Linux.

Another potential problem is if you try to use blocksizes > 64K.  I don't
know if that's still a problem in newer Windows versions, but with older
versions including Windows XP, the OS didn't handle blocksizes > 64K
correctly and we got spurious error messages.  Something about this
should be in the mailing list archives of old.

But the bottom line is, I have no way to test and debug that, since I
don't have access to an LTO-5 drive, nor do I have a Windows machine
with SAS controller.  However, since Cygwin as well as the mt tool are
Open Source, maybe you can have a look and debug this issue?


Thanks,
Corinna

-- 
Corinna Vinschen                  Please, send mails regarding Cygwin to
Cygwin Maintainer                 cygwin AT cygwin DOT com
Red Hat

Attachment: pgp4wMoZe4Eh9.pgp
Description: PGP signature