Re: "Enhanced" MD code avaible for review

[Bartlomiej Zolnierkiewicz <B.Zolnierkiewicz@elka.pw.edu.pl>]

On Thursday 18 of March 2004 01:23, Scott Long wrote:
> Bartlomiej Zolnierkiewicz wrote:
> > On Wednesday 17 of March 2004 22:18, Scott Long wrote:
> >  > Jeff Garzik wrote:
> >  > > Justin T. Gibbs wrote:
> >  > >  > [ I tried sending this last night from my Adaptec email address
> >  > >  > and have yet to see it on the list.  Sorry if this is dup for any
> >  > >  > of
> >
> > you.
> >
> >  > >  > ]
> >  > >
> >  > > Included linux-kernel in the CC (and also bounced this post there).
> >  > >
> >  > >  > For the past few months, Adaptec Inc, has been working to
> >
> > enhance MD.
> >
> >  > > The FAQ from several corners is going to be "why not DM?", so I
> >  > > would humbly request that you (or Scott Long) re-post some of that
> >  > > rationale here...
> >
> > This is #1 question so... why not DM?  8)
> >
> > Regards,
> > Bartlomiej
>
> The primary feature of any RAID implementation is reliability.
> Reliability is a surprisingly hard goal.  Making sure that your
> data is available and trustworthy under real-world scenarios is
> a lot harder than it sounds.  This has been a significant focus
> of ours on MD, and is the primary reason why we chose MD as the
> foundation of our work.

Okay.

> Storage is the foundation of everything that you do with your
> computer.  It needs to work regardless of what happened to your
> filesystem on the last crash, regardless of whether or not you
> have the latest initrd tools, regardless of what rpms you've kept
> up to date on, regardless if your userland works, regardless of
> what libc you are using this week, etc.

I'm thinking about initrd+klibc not rpms+libc,
fs is a lower level than DM - fs crash is not a problem here.

> With DM, what happens when your initrd gets accidentally corrupted?

The same what happens when your kernel image gets corrupted,
probability is similar.

> What happens when the kernel and userland pieces get out of sync?

The same what happens when your kernel driver gets out of sync.

> Maybe you are booting off of a single drive and only using DM arrays
> for secondary storage, but maybe you're not.  If something goes wrong
> with DM, how do you boot?

The same what happens when "something" wrong goes with kernel.

> Secondly, our target here is to interoperate with hardware components
> that run outside the scope of Linux.  The HostRAID or DDF BIOS is
> going to create an array using it's own format.  It's not going to
> have any knowledge of DM config files, initrd, ramfs, etc.  However,

It doesn't need any knowledge of config files, initrd, ramfs etc.

> the end user is still going to expect to be able to seamlessly install
> onto that newly created array, maybe move that array to another system,
> whatever, and have it all Just Work.  Has anyone heard of a hardware
> RAID card that requires you to run OS-specific commands in order to
> access the arrays on it?  Of course not.  The point here is to make
> software raid just as easy to the end user.

It won't require user to run any commands.

RAID card gets detected and initialized -> hotplug event happens ->
user-land configuration tools executed etc.

> The third, and arguably most important issue is the need for reliable
> error recovery.  With the DM model, error recovery would be done in
> userland.  Errors generated during I/O would be kicked to a userland
> app that would then drive the recovery-spare activation-rebuild
> sequence.  That's fine, but what if something happens that prevents
> the userland tool from running?  Maybe it was a daemon that became
> idle and got swapped out to disk, but now you can't swap it back in
> because your I/O is failing.  Or maybe it needs to activate a helper
> module or read a config file, but again it can't because i/o is

I see valid points here but ramfs can be used etc.

> failing.  What if it crashes.  What if the source code gets out of sync
> with the kernel interface.  What if you upgrade glibc and it stops
> working for whatever unknown reason.

glibc is not needed/recommend here.

> Some have suggested in the past that these userland tools get put into
> ramfs and locked into memory.  If you do that, then it might as well be
> part of the kernel anyways.  It's consuming the same memory, if not
> more, than the equivalent code in the kernel (likely a lot more since
> you'd  have to static link it).  And you still have the downsides of it
> possibly getting out of date with the kernel.  So what are the upsides?

Faster/easier development - user-space apps don't OOPS. :-)
Somebody else than kernel people have to update user-land. :-)

> MD is not terribly heavy-weight.  As a monolithic module of
> DDF+ASR+R0+R1 it's about 65k in size.  That's 1/2 the size of your
> average SCSI driver these days, and no one is advocating putting those

SCSI driver is a low-level stuff - it needs direct hardware access.

Even 65k is still a bloat - think about vendor kernel including support
for all possible RAID flavors.  If they are modular - they require initrd
so may as well be put to user-land.

> into userland.  It just doesn't make sense to sacrifice reliability
> for the phantom goal of 'reducing kernel bloat'.

ATARAID drivers are just moving in this direction...
ASR+DDF will also follow this way... sooner or later...

Regards,
Bartlomiej