Re: [RFC 2/7] fs: Add the ZUF filesystem to the build + License

From: Andreas Dilger <adilger@dilger.ca>
To: Boaz Harrosh <boazh@netapp.com>
Cc: linux-fsdevel <linux-fsdevel@vger.kernel.org>,
	Ric Wheeler <rwheeler@redhat.com>,
	Miklos Szeredi <mszeredi@redhat.com>,
	Steve French <smfrench@gmail.com>,
	Steven Whitehouse <swhiteho@redhat.com>,
	Jefff moyer <jmoyer@redhat.com>, Sage Weil <sweil@redhat.com>,
	Jan Kara <jack@suse.cz>, Amir Goldstein <amir73il@gmail.com>,
	Andy Rudof <andy.rudoff@intel.com>,
	Anna Schumaker <Anna.Schumaker@netapp.com>,
	Amit Golander <Amit.Golander@netapp.com>,
	Sagi Manole <sagim@netapp.com>,
	Shachar Sharon <Shachar.Sharon@netapp.com>
Subject: Re: [RFC 2/7] fs: Add the ZUF filesystem to the build + License
Date: Wed, 14 Mar 2018 22:21:07 -0600	[thread overview]
Message-ID: <F8728BF8-D963-481C-96A1-74C1D82334F3@dilger.ca> (raw)
In-Reply-To: <c2133d8c-dbfa-726d-6da0-9f1dc9e702e3@netapp.com>

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On Mar 14, 2018, at 11:21 AM, Boaz Harrosh <boazh@netapp.com> wrote:
> 
> On 13/03/18 22:16, Andreas Dilger wrote:
> <>
>>> + */
>>> +#define ZUFS_MINORS_PER_MAJOR	1024
>>> +#define ZUFS_MAJOR_VERSION 1
>>> +#define ZUFS_MINOR_VERSION 0
>> 
>> I haven't really been following this development, but my recommendation
>> would be to use feature flags (e.g. at least __u64 compat, __u64 incompat)
>> for the API and separately for the disk format, rather than using version
>> numbers.  This makes it clear what "version" relates to a specific feature,
>> and also allows *removal* of features if they turn out to be a bad idea.
>> With version numbers you can only ever *add* features, and have to keep
>> support for every old feature added.
>> 
>> Also, having separate feature flags allows independent development of new
>> features, and doesn't require that feature X has to be in version N or it
>> will break for anyone using/testing that feature outside of the main tree.
>> 
>> This has worked for 25 years for ext2/3/4 and 15 years for Lustre.  ZFS
>> has a slightly more complex feature flags, distinguishing between features
>> that _could_ be used (i.e. enabled at format time or by the administrator),
>> and features that _are_ used (with a refcount).  That avoids gratuitous
>> incompatibility if some feature is enabled, but not actually used (e.g.
>> ext4 files over 2TB), and also allows removing that incompatibility if the
>> feature is no longer used (e.g. all > 2TB files are deleted).
>> 
> 
> Yes thank you. As you can see at this RFC stage I have not even put any
> code to enforce the ABI/API versioning yet. Exactly because I don't like
> it as you explained. I will think about your suggestion and see. This is
> not on disk stuff. This is more the communication channel between
> ZUF<=>ZUS. Though there are a couple on disk stuff.
> (The on disk things are all hidden from here inside the usermode FS plugin)
> 
> The thing is that I want to work a system with the distro's that the
> ZUF<=>ZUS ABI can freely change, by forcing the zusd package be dependent
> on the kernel package. And it be signed by the Kernel's make key. Meaning
> it will only run against the kernel it was compiled against.

That is a major pain, and even the distros are doing things like weak module
symbol versions so that external kernel modules do not need to be rebuilt
for every minor kernel update.

> And keep the stable ABI with feature and versioning between the
> ZUSD<=>zusFS-plugin(s)
> We'll have to see
> 
> Thanks
> Boaz

Cheers, Andreas

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