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

[Bryan J. Smith]

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.

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.

In Linux, the Logical Volume Manager (LVM) disk label offers all
sorts of meta-data store like LDM disk labels, although unlike
Microsoft in NT5+, Linux doesn't make its use mandatory.  The kernel
itself can associate slices in LVM volumes with MD volumes, hence why
I always use MD atop of LVM.  Going the other way, LVM atop of MD is
a nightmare -- especialy if the MD disk cannot associate with other,
required disks in the volume.

As "nicer/easier" it seems to make one big MD volume and then put LVM
atop of it, it's the least stable/reliable configuration.  That's why
LVM+MD gets a bad name, from LVM atop of MD.  In reality, MD atop of
LVM is the recommended solution in my book -- never "lost" a MD
volume myself that way.  Although DeviceMapper now removes the
requirement for the separate MD, and it's completely dynamic,
bootable and recoverable right in GRUB/kernel itself -- no userspace
tools required.


-- 
Bryan J. Smith   Professional, Technical Annoyance
b.j.smith at ieee.org    http://thebs413.blogspot.com
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     Fission Power:  An Inconvenient Solution