[PVE-User] need Help (Hardware Raid)

Muhammad Yousuf Khan sirtcp at gmail.com
Wed May 25 07:45:08 CEST 2011


Thanks you Matthew :) your emails is life saving.

Thanks again,

MYK


On Tue, May 24, 2011 at 6:47 PM, Matthew Caron <Matt.Caron at sixnet.com>wrote:

> On 05/24/2011 09:10 AM, Muhammad Yousuf Khan wrote:
>
>> tell any one :P . have u tested mdam fail over
>>
>
> Yes - it works the same as it does in all the other boxes. If I yank out a
> drive, it just keeps running.
>
> All that said, I *don't* recommend doing software RAID for anything other
> than RAID1. In this respect, I agree with them.
>
> The argument basically boils down like this (and I am paraphrasing from
> forum conversations):
>
> Q: Do you support software RAID?
> A: No. Software RAID is bad because if you lose power it can corrupt your
> disks. Get a battery backed hardware RAID controller.
> Q: But if you are just doing a RAID1? mirror and lose power, it's the same
> as if you're using a single disk.
> A: But if you are doing software RAID5?, it's worse.
> Q: I'm not talking RAID5?, I'm talking RAID1?.
> A: But if we put in support for RAID1?, we'd want to support all software
> RAID levels, otherwise it's a halfway job. So, we're not going to support
> any software RAID.
>
> However, it's still Linux, and it's still Debian, and it's still based off
> the same tools we're used to using, so we can do what we want.
>
> There is even a mostly correct post here:
>
> http://forum.proxmox.com/threads/838-Install-on-Soft-Raid?p=4760#post4760
>
> Here is my (somewhat revised) procedure pulled from my system setup notes:
>
> 1. Log in as root (ssh or console, doesn't matter. ssh is likely easier).
> Note that at this point there is only the root account. Password is what you
> set up in the install.
>
> 2. Get the tools you need:
>
> apt-get install mdadm initramfs-tools
>
> 3. In the mdadm config screen, choose "all" (the default)
>
> 4. Edit the initramfs module list in /etc/initramfs-tools/modules and add
> raid1 at the bottom
>
> 5. sfdisk the drives. Note that these drives have a funky layout, hence the
> --force parameter
>
> sfdisk -d /dev/sda | sfdisk --force /dev/sdb
>
> 6. Create the two metadisks, level 1 (mirrored), 2 devices, first one
> missing. Also bootstraps config.
>
> mdadm --create /dev/md0 --level=1 --raid-devices=2 missing /dev/sdb1
> mdadm --create /dev/md1 --level=1 --raid-devices=2 missing /dev/sdb2
> mdadm --detail scan >> /etc/mdadm/mdadm.conf
>
> 7. Create a new physical lvm volume, extend the existing logical volume
> into it, move the data over, then reduce the existing volume out of the
> array. (Note that the move takes a while.)
>
> pvcreate /dev/md1
> vgextend pve /dev/md1
> pvmove /dev/sda2 /dev/md1
> vgreduce pve /dev/sda2
>
> 8. Add the drive you just reduced out of the array to the metadisk as the
> missing component and then watch the rebuild until it completes
>
> mdadm --add /dev/md1 /dev/sda2
> watch -n1 "cat /proc/mdstat"
>
> 9. Make a filesystem on the smaller metadisk and copy /boot to it
>
> mkfs.ext3 /dev/md0
> mkdir /mnt/md0
> mount /dev/md0 /mnt/md0
> cp -a /boot/* /mnt/md0/.
> umount /mnt/md0
> rmdir /mnt/md0
> umount /boot
>
> 10. edit /etc/fstab and change the line mounting /dev/sda1 as /boot to
> mount /dev/md0 as /boot
>
> 11. Remount our boot drive
>
> mount /boot
>
> 12. change the id of /dev/md0 to fd (linux raid autodetect)
>
> sfdisk --change-id /dev/sda 1 fd
>
> 13. Add the old /boot drive to the metadisk
>
> mdadm --add /dev/md0 /dev/sda1
>
> The rebuild will be basically instant, so you shouldn't have too much of a
> problem. I'd cat /proc/mdstat just to be sure.
>
> 14. Set up grub on the new drive
> grub
> root (hd1,0)
> setup (hd1)
>
> 15. Update the initial ramdisk to reflect all the changes you made:
>
> update-initramfs -t -u
>
> 16. Reboot. If it breaks, you get to keep the pieces.
>
>  and proxmox up-gradation?
>>
>
> I've just been doing standard upgrades with:
> sudo aptitude update
> sudo aptitude full-upgrade
>
> The only bump is sometimes the initramfs doesn't always get set up with the
> correct bits, so you end up having to do:
>
> update-initramfs -k <version> -v -u -t
>
> But this was only a problem in older kernels - I haven't had a problem
> recently.
>
> > is it working fine?
>
> It has been working fine for about a year and a half.
>
>
>  if i go for the software RAID "mdam" how stable it would be
>>
>
> As stable as a single disk. I have no concerns about RAID1 in this regard.
> Writes are simultaneous, reads are round-robin. So, if the power gets cut,
> your disks will be in the same shape as they would have been if there was
> only one - with all the good and bad which that entails.
>
> I have a bit more concern about other RAID levels - since there are parity
> calculations, etc. a battery-backed setup is really the way to go there.
>
>
>  what you say about the performance?
>>
>
> Slightly faster than a single disk on read, the same on write, and low
> overhead. Again, there are no calculations to be done, so there is just a
> bit more additional traffic on the bus. However, the PCIe bus is SO much
> faster than the individual SATA channels that I'm unconcerned. This used to
> be more of an issue with it was, back when you could easily saturate a
> straight PCI bus, but those speeds have so far outstripped the performance
> of rotational media that I wouldn't even consider it an issue unless you're
> using SSD's, and even then you'd have to run the numbers and see.
>
>
>  are you running this in production?
>>
>
> Depends on your definition. We have it as a process-critical server used
> for a variety of development, build, and testing servers (nightly builds and
> testing, etc.). That said, it is not customer facing - it is internal to
> engineering. So, folks would notice if it blew up, and it would make life
> difficult, but if it took me 48 hours to get it back, I could.
>
> As an aside, it's a fairly small rig - Core i7 @ 2.93GHz, 12GB RAM,
> mirrored 500GB HDDs. There are 8 machines provisioned, with 5 active. Most
> of them are only allocated 1GB of memory, one has 2GB, and the aggregate
> Physical memory usage is currently 5.41GB used. CPU Usage is about 3%, with
> a load of about 0.3.
>
> So, the machine is bored most of the time.
>
>
>  please share some of your experience regarding mdam in proxmox as it is
>> not
>> recommended.
>>
>
> Yes, and in some cases, they are correct, and I have no argument. In other
> cases, I respectfully disagree. This is one of those cases.
>
>
>  i am sending this email only to you because PVE community might
>> get angry if we try to legalize  some thing they already declared
>> as unsuitable strategy for proxmox :)
>>
>
> I appreciate that, but I'm putting it back on-list, because I think there's
> more folks out there doing this than many folks realize. I think there is a
> valid use case here, and the project maintainers should be aware of it.
>
> However, they may also have a different opinion, and you're better off
> getting both sides of the story. Someone could call me a fool and cite some
> data of which I was not aware, which could help prevent you from making a
> mistake.
>
> Besides - we are part of the community.
>
> --
> Matthew Caron
> Build Engineer
> Sixnet | www.sixnet.com
> O +1 518 877 5173 Ext. 138
> F +1 518 602 9209
> matt.caron at sixnet.com
>
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