Re: Re: [gentoo-user] raid/partition question

Re: Re: [gentoo-user] raid/partition question

[brettholcomb]

As an extension of this question since I'm working on setting up a system now.  

What is better to do with LVM2 after the RAID is created.  I am using EVMS also.

1.  Make all the RAID freespace a big LVM2 container and then and then create LVM2 volumes on top of this big container.

or 

2.  Parcel out the RAID freespace into LVM2 containers for each partiton (/, /user, etc.).



> 
> From: "Richard Fish" <bigfish@asmallpond.org>
> Date: 2006/02/20 Mon AM 11:04:55 EST
> To: gentoo-user@lists.gentoo.org
> Subject: Re: [gentoo-user] raid/partition question
> 
> On 2/20/06, Nick Smith <nick.smith79@gmail.com> wrote:
> > i think im confusing myself here. can you partition a raid device aka
> > /dev/md0?
> 
> Yes.  You can either use mdadm to create a partitionable raid device,
> or use LVM/EVMS (which would be my recommendation) to create logical
> volumes on the array.
> 
> Just beware that /boot should either be it's own partition (non-raid),
> or on a RAID-1 array (with no partitions).  Otherwise the boot loader
> will have trouble locating and loading the kernel.
> 
> -Richard
> 
> -- 
> gentoo-user@gentoo.org mailing list
> 
> 

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Re: [gentoo-user] raid/partition question

[Boyd Stephen Smith Jr.]

On Monday 20 February 2006 11:51, brettholcomb@bellsouth.net wrote about 
'Re: Re: [gentoo-user] raid/partition question':
> As an extension of this question since I'm working on setting up a
> system now.
>
> What is better to do with LVM2 after the RAID is created.  I am using
> EVMS also.
>
> 1.  Make all the RAID freespace a big LVM2 container and then and then
> create LVM2 volumes on top of this big container.
>
> or
>
> 2.  Parcel out the RAID freespace into LVM2 containers for each partiton
> (/, /user, etc.).

3. Neither.  See below.  First a discussion of the two options.

1. Is fine, but it forces you to choose a single raid level for all your 
data.  I like raid 0 for filesystems that are used a lot, but can easily 
be reconstructed given time (/usr) and especially filesystems that don't 
need to be reconstructed (/var/tmp), raid 5 or 6 for large filesystems 
that I don't want to lose (/home, particularly), and raid 1 for critical, 
but small, filesystems (/boot, maybe).  

2. Is a little silly, since LVM is designed so that you can treat multiple 
pvs as a single pool of data OR you can allocate from a certain pv -- 
whatever suits the task at hand.  So, it rarely makes sense to have 
multiple volume groups; you'd only do this when you want a fault-tolerant 
"air-gap" between two filesystems.

Failure of a single pv in a vg will require some damage control, maybe a 
little, maybe a lot, but having production encounter any problems just 
because development had a disk go bad is unacceptable is many 
environments.  So, you have a strong argument for separate vgs there.

3. My approach: While I don't use EVMS (the LVM tools are fine with me, at 
least for now) I have a software raid 0 and a hw raid 5 as separate pvs in 
a single vg.  I create and expand lvs on the pv that suits the data.  I 
also have a separate (not under lvm) hw raid 0 for swap and hw raid 6 for 
boot.  I may migrate my swap to LVM in the near future; during my initial 
setup, I feared it was unsafe.  Recent experience tells me that's (most 
likely) not the case.

For the uninitiated, you can specify the pv to place lv data on like so:
lvcreate -L <size> -n <name> <vg> <pv>
lvresize -L <size> <vg>/<lv> <pv>
The second command only affect where new extents are allocated, it will not 
move old extents; use pvmove for that.

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ICQ: 514984 YM/AIM: DaTwinkDaddy
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[gentoo-user] raid/partition question

[Nick Smith]

just wanted to ask before i mess something up.
i have booted off the install cd, created a raidtab with my mirrored
drives on it. i have created the raid.  now, do i go in and setup the
partitions i want on that raid? or should i have done that before
creating the raid? so instead of having one big mirror and then
partitioning that, do i need to create my seperate partitions and then
mark them as "fd" and then create each raid seperate?

i think im confusing myself here. can you partition a raid device aka
/dev/md0? when i look at it in fdisk it shows the partitions of both
drives (they are the same).  if i partition md0 and try to write the
table i get :

WARNING: Re-reading the partition table failed with
error 22: Invalid argument.
The kernel still uses the old table.
The new table will be used at the next reboot.
Syncing disks.

did i just set myself up for failure? or is it going to work? im doing
this install remotely so i really cant reboot to see if its going to
work, but i dont want to continue with the install if im just wasting
my time. thats why im asking here to be sure. this is on sparc
hardware, but i dont think that will make a difference.

TIA

Nick

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Re: [gentoo-user] raid/partition question

[Richard Fish]

On 2/20/06, Nick Smith <nick.smith79@gmail.com> wrote:
> i think im confusing myself here. can you partition a raid device aka
> /dev/md0?

Yes.  You can either use mdadm to create a partitionable raid device,
or use LVM/EVMS (which would be my recommendation) to create logical
volumes on the array.

Just beware that /boot should either be it's own partition (non-raid),
or on a RAID-1 array (with no partitions).  Otherwise the boot loader
will have trouble locating and loading the kernel.

-Richard

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Re: [gentoo-user] raid/partition question

[Boyd Stephen Smith Jr.]

On Monday 20 February 2006 09:57, "Nick Smith" <nick.smith79@gmail.com> 
wrote about '[gentoo-user] raid/partition question':
> just wanted to ask before i mess something up.
> i have booted off the install cd, created a raidtab with my mirrored
> drives on it. i have created the raid.  now, do i go in and setup the
> partitions i want on that raid? or should i have done that before
> creating the raid? so instead of having one big mirror and then
> partitioning that, do i need to create my seperate partitions and then
> mark them as "fd" and then create each raid seperate?

I would suggest partitioning the drives identically, then using mdadm to 
create your raid devices.  The reason I say this is because the kernel 
does not seem to have any room in the device node space for partitions on 
an md device.

I could be wrong here; but I know partition and then build will work.

If you'll look at the major/minor number of IDE devices, you'll see that 
hda and hdb have the same major, but the minor number on hdb is +64... 
thus this allows 63 recognized partitions / disk labels on an IDE device. 
(hda1 is +1 minor from hda, hda2 is +2, etc.; similarly for hdb)

If you do the same investigation on SCSI/SATA devices, you'll see that sda 
and sdb have the same major number, but the minor number on sdb is +16... 
thus only 15 partitions / disk labels are recognized on a SCSI/SATA 
device.  I do believe we recently had a member of gentoo-user run into 
this problem.  (Switching to not using partitions as much will help; I 
prefer LVM LVs myself.)

Finally, you can look at the software raid devices, you'll see that md0 and 
md1 have the same major number (9) and the minor number on md1 (1) is only 
+1 from the minor number on md0 (0).  Due do this, I fear that the kernel 
may not properly recognize partitions / disk labels on software raid 
devices.

It's entirely possible that partitions on software raid devices use a 
different major number and/or use dynamic minor numbers so partitioning 
the raid device may work -- I just can't recommend it because I don't know 
it'll work and I know partitioning first, then raid-ing the partitions 
does work.

As the other poster said, be careful with how you treat your bootable 
partition.  It must be a partition recognized by your bootloader, on a 
disk recognized by the BIOS / EMI, using a filesystem understood by your 
bootloader.  If you use the old-style software raid (no superblock; by 
default mdadm does create a superblock), you can use raid 1 for boot, but 
each component partition should satisfy all the conditions for a bootable 
partition.

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ICQ: 514984 YM/AIM: DaTwinkDaddy
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