On 12/05/2014 01:50 PM, Jeff Boyce wrote:
----- Original Message ----- From: "Mark Milhollan" <mlm@xxxxxxxxxxxxx>
To: "Jeff Boyce" <jboyce@xxxxxxxxxxxxxxx>
Sent: Thursday, December 04, 2014 7:18 AM
Subject: Re: CentOS 7 install software Raid on large drives error
On Wed, 3 Dec 2014, Jeff Boyce wrote:
I am trying to install CentOS 7 into a new Dell Precision 3610. I have
two 3
TB drives that I want to setup in software RAID1. I followed the guide
here
for my install as it looked fairly detailed and complete
(http://www.ictdude.com/howto/install-centos-7-software-raid-lvm/).
I suggest using the install guide rather than random crud. The storage
admin guide is fine to read too, but go back to the install guide when
installing.
/mark
Well I thought I had found a decent guide that wasn't random crud, but I
can see now that it was incomplete. I have read the RHEL installation
guide (several times now) and I am still not quite sure that it has all the
knowledge I am looking for.
I have played around with the automated and the manual disk partitioning
system in the installation GUI numerous times now trying to understand what
it is doing, or more accurately, how it responds to what I am doing. I
have made a couple of observations.
1. The installer requires that I have separate partitions for both /boot
and /boot/efi. And it appears that I have to have both of these, not just
one of them.
2. The /boot partition can not reside on LVM.
3. The options within the installer then appear to allow me to create my
LVM with Raid1, but the /boot and /boot/efi are then outside the Raid.
4. It looks like I can set the /boot partition to be Raid1, but then it is
a separate Raid1 from the LVM Raid1 on the rest of the disk. Resulting in
two separate Raid1s; a small Raid1 for /boot and a much larger Raid1 for
the LVM volume group.
I finally manually setup a base partition structure using GParted that
allowed the install to complete using the format below.
sda (3TB)
sda1 /boot fat32 500MB
sda2 /boot/efi fat32 500MB
sdb (3TB)
sdb1 /boot fat32 500MB
sdb2 /boot/efi fat32 500MB
The remaining space was left unpartitioned in GParted, which was then
prepared as LVM Raid1 in the CentOS installer. The installer also put the
/boot and /boot/efi files on sda1 and sda2. Then I would have to manually
copy them over to sdb1 and sdb2 if I wanted to be able to boot from drive
sdb if drive sda failed.
I am not sure that this result is what I really want, as it doesn't Raid my
entire drives. The structure below is what I believe I want to have.
sda & sdb RAID1 to produce md1
md1 partitioned
md1a /boot non-LVM
md1b /boot/efi non-LVM
md1c-f LVM containing /, /var, /home, and /swap
Well the abbreviations may not be the proper syntax, but you probably get
the idea of where I am going. If this is correct, then it looks like I
need to create the RAID from the command line of a rescue disk and set the
/boot and /boot/efi partitions first before beginning the installer. But
then again I could be totally off the mark here so I am looking for someone
to set me straight. Thanks.
Jeff
The last time I actually needed to do this was probably Centos 5, so
someone will correct me if I have not kept up with all the changes.
1. Even though GRUB2 is capable of booting off of an LVM drive, that
capability is disabled in RHEL & Centos. Apparently RH doesn't feel it is
mature yet. Therefore, you need the separate boot partition. (I have a
computer running a non-RH grub2 installation, and it boots off of LVM OK,
but apparently it falls into the "works for me" category).
2. I cannot comment from experience about the separate drive for /boot/efi,
but needing a separate partition surprises me. I have not read about
others needing that. I would think that having an accessible /boot
partition would suffice.
3. When grub (legacy or grub2) boots off of a RAID1 drive, it doesn't
"really" boot off of the RAID. I just finds one of the pair, and boots off
of that "half" of the RAID. It doesn't understand that this is a RAID
drive, but the disk structure for RAID1 is such that it just looks like a
regular drive to GRUB. Basically, it always boots off of sda1. If sda
fails, you have to physically (or in BIOS) swap sda and sdb in order for
grub to find the RAID copy.
4. At one time, I recall that the process for setting up RAID for the boot
drive was basically:
a. Create identical boot partitions on both drives (used to have to be
at the beginning of the drive, I don't think that is necessary any more).
b. Partition the rest of your drive as desired.
c. Do the install using sda1 as the boot partition (ignore sdb1).
d. After the installation, convert sda1 and sdb1 into a RAID1 array
(probably md1 in your case).
e. Go through a process that copies the boot sector information from
sda to sdb, so sdb is ready for the scenario mentioned is step 3.
In summary: grub doesn't understand RAID arrays, but it can be tricked into
booting off of a RAID1 disk partition. However you don't get full RAID
benefits. Yes, you have a backup copy, but grub doesn't know it is there.
It's more like you have to put it in grub's way, so that grub trips over
it and uses it.
The only way to find out if your setup has all the pieces in place is to
physically remove sda, and see if the boot off of sdb completes or not.
Ted Miller
Indiana, USA
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