[Stgt-devel] Dealing with filemarks in SSC ...

[Mark Harvey]

No, an AT&T unix 'close' will position after the filemark... The bsd
unix close positions before the filemark.

Hence if you use the wrong device (mainly a Solaris thing) the backup
s/w gets confused due to multiple filemarks between data.
e.g.
/dev/rmt/0cn (at&t close)
Vs
/dev/rmt/0cbn (bsd close)

With Linux, you need to perform some sort of ioctl() to change the
device close behavior Thankfully, Linux defaults to bsd behaviour.

Cheers
Mark

-----Original Message-----
From: Richard Sharpe [mailto:realrichardsharpe at gmail.com] 
Sent: Wednesday, July 30, 2008 10:17 AM
To: Mark Harvey
Subject: Re: [Stgt-devel] Dealing with filemarks in SSC ...

On Tue, Jul 29, 2008 at 4:54 PM, Mark Harvey <mark_harvey at symantec.com>
wrote:
> Both NetBackup and NetWorker use the following filemark handling.
>
> [data][filemark1][data][filemark1][data][filemark1][filemark2]
>
> Using the 'bsd' close (vs AT&T close), the tape head is positioned
after
> [filemark1] and before [filemark2]. A new 'write' command will
overwrite
> [filemark2].

So, reading between the lines, if you have been writing, a BSD close
writes two filemarks and then positions the tape between them ...

> Using this method, end of valid backup data is easily detected by two
> consecutive filemarks on tape. If there is only one filemark, there is
> more valid data to follow.
>
> Note: filemark1 and filemark2 are just 'filemarks' - they are not
> different types of filemarks. (Just in case this created any sort of
> confusion).
>
> Cheers
> Mark
>
> -----Original Message-----
> From: stgt-devel-bounces at lists.berlios.de
> [mailto:stgt-devel-bounces at lists.berlios.de] On Behalf Of Richard
Sharpe
> Sent: Wednesday, July 30, 2008 9:45 AM
> To: stgt-devel
> Subject: [Stgt-devel] Dealing with filemarks in SSC ...
>
> I have been looking at this issue of filemarks in SSC, and it seems to
> me that there are a couple of questions that need to be answered.
>
> For example, if there are two file marks on a tape, or even three, and
> you write records onto the tape and overwrite one of the file marks,
> in some sense the drive should still be able to seek to the old second
> filemark (now the first filemark) ... and reliably recover the third
> file on the tape.
>
> I don't know if any software depends on that behavior, though.
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