Re: [PATCH 7/9] exofs: mkexofs

[James Bottomley]

On Sun, 2009-01-04 at 17:20 +0200, Boaz Harrosh wrote:
> James Bottomley wrote:
> > On Wed, 2008-12-31 at 17:19 +0200, Boaz Harrosh wrote:
> >> Andrew Morton wrote:
> >>> On Tue, 16 Dec 2008 17:33:48 +0200
> >>> Boaz Harrosh <bharrosh@xxxxxxxxxxx> wrote:
> >>>
> >>>> We need a mechanism to prepare the file system (mkfs).
> >>>> I chose to implement that by means of a couple of
> >>>> mount-options. Because there is no user-mode API for committing
> >>>> OSD commands. And also, all this stuff is highly internal to
> >>>> the file system itself.
> >>>>
> >>>> - Added two mount options mkfs=0/1,format=capacity_in_meg, so mkfs/format
> >>>>   can be executed by kernel code just before mount. An mkexofs utility
> >>>>   can now be implemented by means of a script that mounts and unmount the
> >>>>   file system with proper options.
> >>> Doing mkfs in-kernel is unusual.  I don't think the above description
> >>> sufficiently helps the uninitiated understand why mkfs cannot be done
> >>> in userspace as usual.  Please flesh it out a bit.
> >> There are a few main reasons.
> >> - There is no user-mode API for initiating OSD commands. Such a subsystem
> >>   would be hundredfold bigger then the mkfs code submitted. I think it would be
> >>   hard and stupid to maintain a complex user-mode API just for creating
> >>   a couple of objects and writing a couple of on disk structures.
> > 
> > This is really a reflection of the whole problem with the OSD paradigm.
> 
> Certainly not a problem of the OSD paradigm, just maybe a problem
> of the current code boundaries laid out by years of block-devices.

Not having a suggestion for redrawing the boundaries is a problem of the
paradigm.  Right at the moment using OSD is an all or nothing, there's
no migration path for block based filesystems, or even a good idea how
they'd take advantage of OSD.  Most OSD based filesystems are for
special purpose things (mainly cluster FS).

> > In theory, a filesystem on OSD is a thin layer of metadata mapping
> > objects to files.  Get this right and the storage will manage things,
> - objects to files.  Get this right and the storage will manage things,
> + files to objects.  Get this right and the storage will manage things,
> [objects to files is what some of the osd-targets do.]
> > like security and access and attributes (there's even a natural mapping
> > to the VFS concept of extended attributes).  Plus, the storage has
> > enough information to manage persistence, backups and replication.
> > 
> 
> Sounds perfect to me.
> 
> > The real problem is that no-one has actually managed to come up with a
> > useful VFS<->OSD mapping layer (even by extending or altering the VFS).
> > Every filesystem that currently uses OSD has a separate direct OSD
> > speaking interface (i.e. it slices out the block layer to do this and
> > talks directly to the storage).
> 
> I'm not sure what you mean.
> Lets take VFS<->BLOCKS mapping for example. Each FS has it's own
> interpretation of what that means, brtfs is less perfect then xfs
> or vice versa?
> I guess you did not mean "mapping" but meant "Interface" or API.
> (or more likely I misunderstand the meaning of "mapping" ;)

No ... by mapping I mean mapping of VFS functions.

For example, an OSD filesystem should be user mountable: if the user has
the security key (could possibly do this in userspace).  Additionally,
an OSD with attributes should be pluggable into the VFS layer
sufficiently to allow attribute search, even if the VFS has no idea of
the metadata layout, we can still get objects back.  We'd also better be
able to do backup and restore of object based devices.

The basic problem for OSD, at least as I see it is that unless it can
provide some compelling relevance to current filesystem problems (like
attribute search is 10x faster over OSD vs block or X filesystem gets a
2x performance improvement using OSD vs block ...) it's doomed forever
to be a niche player: nice idea but no relevance to the real world.

> Well that is exactly what I was attempting to submit. A general-purpose
> low-level but easy-to-use, objects API for kernel clients. be it a
> dead-simple exofs, or a complex multi-head beast like a pNFS-Objects
> file system. The same library/API/Interface will be used for NFS-Clients
> NFSD-Servers, reconstruction, security what ever.

OK ... perhaps I missed the description of how a general purpose
filesystem might use this then?

> The block-layer is not sliced out, Only the elevator function is, since
> BIO merging, if any, are not device global but per-object/file, and the
> elevator does not currently support that. (Profiling shows that it will
> be needed)

Um, your submission path is character.  You pick up block again because
SCSI uses it for queues, but it's not really part of your paradigm.

> BTW. The block-based filesystems are just a big minority in Kernel. The
> majority does not use block-layer either.
> 
> > 
> > I suppose this could be taken to show that such a layer is impossibly
> > complex, as you assert, but its lack is reflected in strange looking
> > design decisions like in-kernel mkfs.  It would also mean that there
> > would be very little layered code sharing between ODS based filesystems.
> - would be very little layered code sharing between ODS based filesystems.
> + would be very little layered code sharing between OSD based filesystems.
> 
> I disagree.
> All the OSD-Based file systems (In Linux) should absolutely only use the
> open-osd library submitted. I myself will work on a couple. If anything is
> missing that could not be added later, I would like to know about it.

But that's precisely the problem:  "OSD based filesystems" implying that
if you want to use OSD you write a new filesystem.

> User-mode Interface is another matter. There are some ideas and some already
> implemented.
> [Hosted on open-osd.org
>  see: http://git.open-osd.org/gitweb.cgi?p=osc-osd/.git;a=summary
>  look inside the osd-initiator directory]
> And I have a toy interface that adds no new entries into the Kernel in
> the form of an OSDVFS module, that will let you access the raw OSD device
> through the VFS name-space.

OK, so this is moving it more towards general usability.

> The lack of any user-mode API is just the lack of any current need/priority,
> or that I'm the only one working on OSD. But nothing that could not be solved
> in two weeks of pragmatic work. Surly it's not a paradigm problem.

It's an indicator of one.  If you buy my premise that OSD cannot be
relevant without compelling user cases, then the lack of a user API can
be viewed as a symptom of this.

> > 
> >> - I intend to refactor the code further to make use of more super.c services,
> >>   so to make this addition even smaller. Also future direction of raid over
> >>   multiple objects will make even more kernel infrastructure needed which
> >>   will need even more user-mode code duplication.
> >> - I anticipate problems that are not yet addressed in this body of work
> >>   but will be in the future, mainly that a single OSD-target (lun) can
> >>   be shared by lots of FSs, and a single FS can span many OSD-targets.
> >>   Some central management is much easier to do in Kernel.
> >>
> >>> What are the dependencies for this filesystem code?  I assume that it
> >>> depends on various block- and scsi-level patches?  Which ones, and
> >>> what is their status, and is this code even compileable without them?
> >>>
> >> This OSD-based file system is dependent on the open-osd initiator library
> >> code that I've submitted for inclusion for 2.6.29. It has been sitting
> >> in linux-next for a while now, and has not been receiving any comments
> >> for the last two updated patchsets I've sent to scsi-misc/lkml. However
> >> it has not yet been submitted into Jame's scsi-misc git tree, and James
> >> is the ultimate maintainer that should submit this work. I hope it will
> >> still be submitted into 2.6.29, as this code is totally self sufficient
> >> and does not endangers or changes any other Kernel subsystems.
> >> (All the needed ground work was already submitted to Linus since 2.6.26)
> >> So why should it not?
> > 
> > I don't like it mainly because it's not truly a useful general framework
> > for others to build on.  However, as argued above, there might not
> > actually be such a useful framework, so as long as the only two
> > consumers (you and Lustre) want an interface like this, I'll put it in.
> > 
> 
> Time will tell, but I believe the exact opposite. I believe and strive
> for this OSD body of work to be useful for anybody that needs to talk
> T10-OSD in Linux, be it for any-purpose. Any thing missing should be
> easily added.
> 
> > James
> > 
> > 
> 
> To summarize the way I see it:
> - James is right in that we can not currently see the full OSD picture since
>   we do not have a user-mode API, so the usefulness of it all is unclear.
>   [I will send an RFD soon, and hope all interested will chime in on the
>    discussion]
> - That said, all the submitted code is still relevant and useful,
>   though at few places it takes the route of pragmatic-easy vs
>   long-term-correctness. [Which can be fixed]
> - exofs/OSD is not the first FS that depends on a none-block-dev/its-own
>   stack. The lower level (OSD) is represented to kernel as a char-dev +
>   Additional API, common to other FS/stack models. Though the lower OSD
>   level has the potential to be a generic layer that can be used by lots
>   of users and use cases, not only FS type.

Right, so I'm reasonably happy to accept libosd for what it is:  an
enabler for a few specialised applications. 

I think your choice of using a character device will turn out to be a
design mistake because the migration path of existing filesystems is
bound to be a block device with extra features (which they may or may
not make use of) but only if there's a way to make ODS relevant to
users.

James


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