linux-ext4 - Re: [PATCH] obsolete libcom-err for SuSE e2fsprogs

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Date:	Tue, 25 Sep 2007 14:34:54 +0200
From:	Karel Zak <kzak@...hat.com>
To:	Kay Sievers <kay.sievers@...y.org>
Cc:	Theodore Tso <tytso@....edu>, Eric Sandeen <sandeen@...hat.com>,
	Andreas Dilger <adilger@...sterfs.com>,
	linux-ext4@...r.kernel.org,
	List util-linux-ng <util-linux-ng@...r.kernel.org>
Subject: Re: [PATCH] obsolete libcom-err for SuSE e2fsprogs

On Tue, Sep 25, 2007 at 12:11:23PM +0200, Kay Sievers wrote:
> On Tue, 2007-09-25 at 11:14 +0200, Karel Zak wrote:
> >  Now we duplicate a lot of FS probe code in libblkid, libvolume_id and
> >  libdisk.
> > 
> >  Yes, it's long-term task, but it's good direction (IMHO).
> 
> It is, and we are not providing a real value to the users by providing
> both of these libs at the same time. :)
> 
> But udev has a very different requirement for probing filesystems.
> Unlike non-udev systems, we can't accept any hidden policy inside a
> library. We just want to pass a byte stream to the lib, and get back
> what exactly is in _this_ byte stream. There must be no chaching, no
> devmapper logic, no stat()'ing in /dev, no reading of /proc/partitions,
> no ioctl()'s, no hidden decisions, nothing. None of these actions is
> acceptable to be done by the library itself, if udev is used. We need
> pure mechanics, no policy. We also need an API that allows to specify
> the size of the stream and the probing offset. And we don't want to
> iterate over tags, need the filesystem version information, the raid
> metadata probing, and the classification volume_id provides.

 Technical details :-)

> That means that we could replace the "low-level" part of libblkid with
> volume_id code, and keep the current API of blkid, but offer some of the
> low-level functionality for udev. Or we would implement the relevant
> parts and probers in blkid, and add a new API to access the low-level
> part directly.

 I don't see a problem to provide low-level and high-level interface in
 same library. The low-level stuff (fsprobe code) is still same.

 Things like a cache are discussable -- for example resolve LABEL/UUID
 on system with udev is faster by /dev/disk/by-*. I think a good
 library has to be smart enough to found the fastest way how translate
 LABEL/UUID to device name.

 But, it's too early to talk about details.

> Let me know what you think how go ahead with this, I'm all for doing
> this sooner than later.

 Cool. I'd like to create libfsprobe as an independent project. Or is
 there any advantage to merge everything to util-linux-ng? I don't
 think so.

    Karel

-- 
 Karel Zak  <kzak@...hat.com>
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