Re: [PATCH 1/6] trace-cmd library: Bump the trace file version to 7

[Steven Rostedt <rostedt@goodmis.org>]

On Thu, 29 Apr 2021 06:43:54 +0300
Tzvetomir Stoyanov <tz.stoyanov@gmail.com> wrote:

> On Thu, Apr 29, 2021 at 4:19 AM Steven Rostedt <rostedt@goodmis.org> wrote:
> >
> > On Thu, 22 Apr 2021 10:17:13 +0300
> > "Tzvetomir Stoyanov (VMware)" <tz.stoyanov@gmail.com> wrote:
> >  
> > > Adding a compression of the trace.dat file will change its structure.
> > > These changes are not backward compatible, the old trace-cmd binaries
> > > will not be able to read compressed trace files. Bumping the version to
> > > 7 will prevent old trace-cmd to read such files.  
> >
> > But this series doesn't add anything to the file that breaks the
> > version. The version should be updated with the patch that breaks the
> > backward compatibility. of the file, not before.
> >  
> I need the new version before the compression changes, because there
> is logic which relies on the new version. There is a check whether to
> read / write compression data based on the new file version. I was
> wondering if to put all changes into a single patchset, version +
> compression. Decided to split in two, although there is no sense to
> bump the version without adding a compression.

The file should stay at version 6 unless it has something that old
trace-cmd can not read.

Right now, with this patch, if I record a trace.dat file, and I port the
version check to older trace-cmd, it will think it can not read this file,
even though there's nothing in this file.

In fact, it should write version 6 until it writes something that is not
supported by version 6.

It will still need to be able to write version 6 files. In fact that's one
of the requirements here. That we allow trace-cmd to create older versions
if it does not use the features of the the new version.

-- Steve