[Pc_Support] Re: WinXP 64-Bit Software Raid Setup Howto? -- How Linux DeviceMapper handles FRAID ...

[Jason Boxman]

Bryan J. Smith wrote:
> Jason Boxman <jasonb at edseek.com> wrote:
>> You speak of this often, but I don't understand.  I haven't
>> personally experienced lost MD volumes, so I don't understand how
>> that will happen to me?
>
> Well I have.
>
> It is especially notorious if a drive does not come up once or twice
> and then is reconnected, or a drive is connected on a different
> channel, etc...  In a nutshell, there is no "dynamic way" to keep
> track of a slice and its volume association outside of the filesystem
> itself.  The volume is then "lost" -- e.g., a "broken mirror" in the
> case of RAID-1 or "lost stripe" in the case of RAID-0.

Okay, I understand what you're talking about now.

I've been booting with an initrd, so I have mdadm proping drives for me on
boot.  Otherwise, I see how I'd lose my system when disks get lost or moved
about and the kernel can't determine what has happened.

> Hence the "common name" Microsoft chose for the Logical Disk Manager
> "LDM" disk label -- the "Dynamic Disk."  It allows for all sorts of
> meta-data to be stored outside of the filesystem as part of the disk
> label itself and associated "dynamically" by a booting kernel that
> has no other info or interogation software.  The legacy BIOS/DOS disk
> does not, and relies on statically configured information.
>
> Again, the mdadm tools have helped quite a bit over the legacy
> raidtools for MD, especially when it comes to re-scanning finding
> those "lost/broken volumes."  But for boot-time operations, that's
> not an option.  Then again, MD is difficult to use with boot-time
> operations -- hence why most /boot filesystems are not located on
> them for safety/usability.

Do you not use mdadm in an initrd at boot time?  With most distro kernels
being of the initrd variety, I'd be surprised to find mdadm wasn't being
included in a kernel if MD support was offered?

Thanks.