Re: [PATCH 7/9] exofs: mkexofs

Jeff Garzik
2009-01-12T20:23:09Z

James Bottomley wrote:
> On Mon, 2009-01-12 at 14:23 -0500, Jeff Garzik wrote:
>>> 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.
>> If having a compelling user case was a prereq for kernel inclusion, well
>> over half the code would be gone.
>
> I'm not holding this against inclusion ... I'm saying it's a symptom of
> the generic relevance to user issues problem that OSD has.
>
>>> 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.
>> It is fantasy to think we will be migrating ext4 to OSD. That fantasy
>> is not a compelling reason to block OSD development.
>
> OK, so your quote managed to miss this bit:
>
> "Right, so I'm reasonably happy to accept libosd for what it is: an
> enabler for a few specialised applications. "
>
> I can't see how that can be construed as "blocking OSD development".
> The word "accept" is conventionally used in Linux parlance to mean "will
> send upstream".

Yet you continue to expend energy complaining about migrating
block-based filesystems to OSD, a complex, overhead-laden undertaking
_no one_ has proposed or entertained.

>> To sum,
>>
>> * exofs needs a userspace library, around which the standard filesystem
>> tools will be built, most notably mkfs, dump, restore, fsck
>>
>> * talk of migrating existing filesystems is wildly premature (and a bit
>> of a silly argument, since you are also arguing that OSD lacks
>> compelling use cases)
>
> So criticising lacking compelling use cases while at the same time
> suggesting how to find them is wrong?
>
> Actually, If the only use case OSD can bring to the table is requiring
> new filesystems, then there's nothing of general user relevance for it
> on the horizon ... anywhere. There's never going to be a compelling
> reason to move the consumer OSDs in the various development labs to
> production because nothing would be able to use them on a mass scale.

> If we could derive a benefit from OSD in existing filesystems, then they
> do have user relevance, and Seagate and the others might just consider
> releasing the devices.

If Seagate were to release a production OSD device, do you really think
they would prefer a block-based filesystem hacked to work with OSDs? I
don't think so.

Existing block filesystems are very much purpose built for sector-based
storage as implemented on modern storage devices. No kernel API can
hand-wave that away.

The whole point of OSDs is to move some of the overhead to the storage
device, not _add_ to the overhead.

> Note that "providing benefit to" does not equate to "rewriting the
> filesystem for" ... and it shouldn't; the benefit really should be
> incremental. And that's the crux of my criticism. While OSD are
> separate things that we have to rewrite whole filesystems for, they're
> never going to set the world on fire. If they could be used with only
> incremental effort, they might. The bridge for the incremental effort
> will come from a properly designed kernel API.

Well, hey, if you wanna expend energy creating a kernel API that
presents a complex OSD as simple block-based storage, go for it. AFAICS
it's just extra overhead and complexity when a new filesystem could do
the job much better.

And I seriously doubt Linus or anyone else will want to hack up a
block-based filesystem in this manner. Better to create a silly "for
argument's sake" OSD block device, upon which any block-based filesystem
can be mounted. (Note I said block device, _not_ filesystem)

>> * an in-kernel OSD-based filesystem needs some sort of generic in-kernel
>> libosd API, so that multiple OSD filesystems do not reinvent the wheel
>> each time.
>>
>> * OSD was bound to be annoying, because it forces the kernel filesystem
>> to either (a) talk SCSI or (b) use messages that can be converted to
>> SCSI OSD commands, like existing drivers convert the block layer's READ
>> and WRITE to device-specific commands.
>
> OK, so what you're arguing is that unlike block devices where we can
> produce a useful generic abstraction that is protocol agnostic, for OSD
> we can't? As I've said before, I think this might be true, but fear it
> dooms OSD to being too difficult to use.

No, a generic abstraction is "(b)" in my quoted paragraph.

But it's certainly easy to create an OSD block device client, that
simulates sector-based storage, if you are motivated in that direction.

But that only makes sense if you want the extra overhead (square peg,
round hole), which no sane person will want. Face it, only screwballs
want to mount ext4 on an OSD.

>> * Trying to force OSD to export a block device is pushing a square peg
>> through a round hole. Thus, the best (and only) alternative is
>> character device. What you really want is a Third Way(tm): a mmap'able
>> message device, since you really want to export an API to userspace.
>
> only allowing a character tap raises the effort bar on getting other
> filesystems to use it, because they're all block based ...

That's irrelevant, since no one is calling for block-based filesystems
to be converted to use OSD.

And I can only imagine the push-back, should someone actually propose
doing so. Filesystems are very much purpose-built for their storage
paradigm.

Jeff