Re: Proposed enhancements to MD

On Thu, Jan 15, 2004 at 10:07:34AM +1100, Neil Brown wrote:

On Monday January 12, scott_long@adaptec.com wrote:

quoted

All,

Adaptec has been looking at the MD driver for a foundation for their
Open-Source software RAID stack.  This will help us provide full
and open support for current and future Adaptec RAID products (as
opposed to the limited support through closed drivers that we have
now).

Sounds like a great idea.

quoted

- Metadata abstraction:  We intend to support multiple on-disk metadata
   formats, along with the 'native MD' format.  To do this, specific
   knowledge of MD on-disk structures must be abstracted out of the core
   and personalities modules.

In 2.4, this would be a massive amount of work and I don't recommend
it.

Scott has a decent stab at doing so already in 2.4, and I've encouraged him
to post the code he's got now.  Since it's too intrusive for 2.4,
perhaps it could be added in parallel, an "emd" driver, and one could
choose to use emd to get the DDF functionality, or continue to use md
without DDF.

Here are some of the features I know I'm looking for, and I've
compared solutions suggested. Comments/corrections welcome.

* Solution works in both 2.4 and 2.6 kernels
  - less ideal of two different solutions are needed
* RAID 0,1 DDF format
* Bootable from degraded R1
* Online Rebuild
* Mgmt tools/hooks
  - online create, delete, modify
* Event notification/logging
* Error Handling
* Installation - simple i.e. without modifying distro installers
  significantly or at all; driver disk only is ideal


From what I see about DM at present:
* RAID 0,1 possible, dm-raid1 module in Sistina CVS needs to get merged
* Boot drive - requires setup method early in boot process, either
  initrd or kernel code
* Boot from degraded RAID1 requires setup method early in boot
  process, either initrd or kernel code.
* Online Rebuild - dm-raid1 has this capability
* mgmt tools/hooks - DM has today way to communicate to kernel the
  changes desired. What remains is userspace tools that read, modify DDF
  metadata and calls into these hooks.
* Event notification / logging - doesn't appear to exist in DM
* Error handling - unclear if/how DM handles this.  For instance, how
  is a disk failure on a dm-raid1 array handled?
* Installation - RHEL3 doesn't include DM yet, significant installer
  work necessary for several distros.


From what I see about md:
* RAID 0,1 there today, no DDF
* Boot drive - yes
* Boot from degraded RAID1 - possible but may require manual
  intervention depending on BIOS capabilities
* Online Rebuild - there today
* mgmt tools/hooks - mdadm there today
* Event notification / logging - mdadm there today
* Error handling - there today
* Installation - disto installer capable of this today


From what I see about emd:
* RAID 0,1 - code being developed by Adaptec today, DDF capable
* Boot drive - yes
* Boot from degraded RAID1 - possible without intervention due to
  Adaptec BIOS
* Online Rebuild - there today
* mgmt tools/hooks - mdadm there today, expect Adaptec to enhance mdam to support DDF
* Event notification / logging - mdadm there today
* Error handling - there today
* Installation - could be done with only a driver disk which adds the
  emd module.

Am I way off base here? :-)

Thanks,
Matt

-- 
Matt Domsch
Sr. Software Engineer, Lead Engineer
Dell Linux Solutions www.dell.com/linux
Linux on Dell mailing lists @ http://lists.us.dell.com

On 2004-01-15T15:52:21,
   Matt Domsch [off-list ref] said:

* Solution works in both 2.4 and 2.6 kernels
  - less ideal of two different solutions are needed

Sure, this is important.

* RAID 0,1 DDF format
* Bootable from degraded R1

We were looking at extending the boot loader (grub/lilo) to have
additional support for R1 & multipath. (ie, booting from the first
drive/path in the set where a consistent image can be read.) If the BIOS
supports DDF too, this would get even better.

For the boot drive, this is highly desireable!

Do you know whether DDF can also support simple multipathing?

* Boot from degraded RAID1 requires setup method early in boot
  process, either initrd or kernel code.

This is needed with DDF too; we need to parse the DDF data somewhere
afterall.

From what I see about md:
* RAID 0,1 there today, no DDF

Supporting additional metadata is desireable. For 2.6, this is already
in the code, and I am looking forward to having this feature.

Am I way off base here? :-)

I don't think so. But for 2.6, the functionality should go either into
DM or MD, not into emd. I don't care which, really, both sides have good
arguments, none of which _really_ matter from a user-perspective ;-)

(If, in 2.7 time, we rip out MD and fully integrate it all into DM, then
we can see further.)


Sincerely,
    Lars Marowsky-Brée [off-list ref]

-- 
High Availability & Clustering	      \ ever tried. ever failed. no matter.
SUSE Labs			      | try again. fail again. fail better.
Research & Development, SUSE LINUX AG \ 	-- Samuel Beckett

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