John Stoffel wrote:
"Michael" == Michael Tokarev <mjt@xxxxxxxxxx> writes:
Michael> Doug Ledford wrote:
Michael> []
1.0, 1.1, and 1.2 are the same format, just in different positions on
the disk. Of the three, the 1.1 format is the safest to use since it
won't allow you to accidentally have some sort of metadata between the
beginning of the disk and the raid superblock (such as an lvm2
superblock), and hence whenever the raid array isn't up, you won't be
able to accidentally mount the lvm2 volumes, filesystem, etc. (In worse
case situations, I've seen lvm2 find a superblock on one RAID1 array
member when the RAID1 array was down, the system came up, you used the
system, the two copies of the raid array were made drastically
inconsistent, then at the next reboot, the situation that prevented the
RAID1 from starting was resolved, and it never know it failed to start
last time, and the two inconsistent members we put back into a clean
array). So, deprecating any of these is not really helpful. And you
need to keep the old 0.90 format around for back compatibility with
thousands of existing raid arrays.
Michael> Well, I strongly, completely disagree. You described a
Michael> real-world situation, and that's unfortunate, BUT: for at
Michael> least raid1, there ARE cases, pretty valid ones, when one
Michael> NEEDS to mount the filesystem without bringing up raid.
Michael> Raid1 allows that.
Please describe one such case please. There have certainly been hacks
of various RAID systems on other OSes such as Solaris where the VxVM
and/or Solstice DiskSuite allowed you to encapsulate an existing
partition into a RAID array.
But in my experience (and I'm a professional sysadm... :-) it's not
really all that useful, and can lead to problems liks those described
by Doug.
If you are going to mirror an existing filesystem, then by definition
you have a second disk or partition available for the purpose. So you
would merely setup the new RAID1, in degraded mode, using the new
partition as the base. Then you copy the data over to the new RAID1
device, change your boot setup, and reboot.
Once that is done, you can then add the original partition into the
RAID1 array.
As Doug says, and I agree strongly, you DO NOT want to have the
possibility of confusion and data loss, especially on bootup. And
this leads to the heart of my initial post on this matter, that the
confusion of having four different variations of RAID superblocks is
bad. We should deprecate them down to just two, the old 0.90 format,
and the new 1.x format at the start of the RAID volume.
Perhaps I am misreading you here, when you say "depreciate them down" do
you mean the Adrian Bunk method of putting in a printk scolding the
administrator, and then remove the feature a version later, or did you
mean "depreciate all but two" which clearly doesn't suggest removing the
capability at all?
--
bill davidsen <davidsen@xxxxxxx>
CTO TMR Associates, Inc
Doing interesting things with small computers since 1979
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