Re: [PATCH 0/3] preserve xattrs in images

[Patrick Ohly <patrick.ohly@intel.com>]

On Tue, 2015-08-11 at 15:29 +0100, Burton, Ross wrote:
> Hi Patrick,
> 
> On 11 August 2015 at 09:44, Patrick Ohly <patrick.ohly@intel.com>
> wrote:
>         The default does not get changed because supporting xattrs
>         causes a
>         certain overhead (need to build GNU tar, additional system
>         calls when
>         creating the images).
> 
> 
> Two questions:
> 1) Do enough host distributions not enable xattrs in tar that we need
> to depend on tar-replacement-native?

Yes, some of the currently supported host distributions (like Debian 7)
have a GNU tar which is too old.

> 2) Have you timed the overhead of enabling xattr archiving on an image
> that doesn't use xattrs?  (subtext: does this need to be an option).

I hadn't, because my concern was that there might be other build
configurations (for example, with less RAM for the page cache) where the
effect will be more pronounced than on my own system.

Anyway, some benchmark data: with a rootfs of around 400MB and 14000
files and directories and xattrs on all regular files, "tar
-cf /dev/null" takes around 1s wall clock time when the entire rootfs
tree is already in the page cache. With archiving xattrs, it is around 3
seconds. If there no xattrs on the file, that last value drops a bit to
something between 2 and 3.

So the relative runtime overhead is noticeable, but it is negligible in
absolute terms.

More important is that GNU tar needs to be built. That takes about a
minute of wall clock time here (most of it in configure, actual
compilation is parallel and fast thanks for many cores).

-- 
Best Regards, Patrick Ohly

The content of this message is my personal opinion only and although
I am an employee of Intel, the statements I make here in no way
represent Intel's position on the issue, nor am I authorized to speak
on behalf of Intel on this matter.