Re: "Enhanced" MD code avaible for review

From: Lincoln Dale
Date: Sat Mar 27 2004 - 19:13:03 EST

At 03:43 AM 27/03/2004, Justin T. Gibbs wrote:
I posted a rather detailed, technical, analysis of what I believe would
be required to make this work correctly using a userland approach. The
only response I've received is from Neil Brown. Please, point out, in
a technical fashion, how you would address the feature set being proposed:

i'll have a go.

your position is one of "put it all in the kernel".
Jeff, Neil, Kevin et al is one of "it can live in userspace".

to that end, i agree with the userspace approach.
the way i personally believe that it SHOULD happen is that you tie your metadata format (and RAID format, if its different to others) into DM.

you boot up using an initrd where you can start some form of userspace management daemon from initrd.
you can have your binary (userspace) tools started from initrd which can populate the tables for all disks/filesystems, including pivoting to a new root filesystem if need-be.

the only thing your BIOS/int13h redirection needs to do is be able to provide sufficient information to be capable of loading the kernel and the initial ramdisk.
perhaps that means that you guys could provide enhancements to grub/lilo if they are insufficient for things like finding a secondary copy of initrd/vmlinuz. (if such issues exist, wouldn't it be better to do things the "open source way" and help improve the overall tools, if the end goal ends up being the same: enabling YOUR system to work better?)

moving forward, perhaps initrd will be deprecated in favour of initramfs - but until then, there isn't any downside to this approach that i can see.

with all this in mind, and the basic premise being that as a minimum, the kernel has booted, and initrd is working
then answering your other points:

o Rebuilds

userspace is running.
rebuilds are simply a process of your userspace tools recognising that there are disk groups in a inconsistent state, and don't bring them online, but rather, do whatever is necessary to rebuild them.
nothing says that you cannot have a KERNEL-space 'helper' to help do the rebuild..

o Auto-array enumeration

your userspace tool can receive notification (via udev/hotplug) when new disks/devices appear. from there, your userspace tool can read whatever metadata exists on the disk, and use that to enumerate whatever block devices exist.

perhaps DM needs some hooks to be able to do this - but i believe that the DM v4 ioctls cover this already.

o Meta-data updates for topology changes (failed members, spare activation)

a failed member may be as a result of a disk being pulled out. for such an event, udev/hotplug should tell your userspace daemon.
a failed member may be as a result of lots of I/O errors. perhaps there is work needed in the linux block layer to indicate some form of hotplug event such as 'excessive errors', perhaps its something needed in the DM layer. in either case, it isn't out of the question that userspace can be notified.

for a "spare activation", once again, that can be done entirely from userspace.

o Meta-data updates for "safe mode"

seems implementation specific to me.

o Array creation/deletion

the short answer here is "how does one create or remove DM/LVM/MD partitions today?"
it certainly isn't in the kernel ...

o "Hot member addition"

this should also be possible today.
i haven't looked too closely at whether there are sufficient interfaces for quiescence of I/O or not - but once again, if not, why not implement something that can be used for all?

Only then can a true comparative analysis of which solution is "less
complex", "more maintainable", and "smaller" be performed.

there may be less lines of code involved in "entirely in kernel" for YOUR hardware --
but what about when 4 other storage vendors come out with such a card?
what if someone wants to use your card in conjunction with the storage being multipathed or replicated automatically?
what about when someone wants to create snapshots for backups?

all that functionality has to then go into your EMD driver.

Adaptec may decide all that is too hard -- at which point, your product may become obsolete as the storage paradigms have moved beyond what your EMD driver is capable of.
if you could tie it into DM -- which i believe to be the defacto path forward for lots of this cool functionality -- you gain this kind of functionality gratis -- or at least with minimal effort to integrate.

better yet, Linux as a whole benefits from your involvement -- your time/effort isn't put into something specific to your hardware -- but rather your time/effort is put into something that can be used by all.

this conversation really sounds like the same one you had with James about the SCSI Mid layer and why you just have to bypass items there and do your own proprietary things. in summary, i don't believe you should be focussing on a short-term viiew of "but its more lines of code", but rather a more big-picture view of "overall, there will be LESS lines of code" and "it will fit better into the overall device-mapper/block-remapper functionality" within the kernel.



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