Re: LVM root?
On Thu, Oct 12, 2006 at 04:35:02PM +0200, Jean-Luc Coulon (f5ibh) wrote:
> *Real* hardware raid doesnt need an OS layer / driver to work.
> This kind of raid relies on the BIOS *and* on a Windows driver.
> It is more a raid feature enabled in the BIOS and managed by the
> Windows driver.
> Linux can or not support this BIOS feature depending of the chipset.
>
> Most of the time, you have to disable the raid in the BIOS and use pure
> software raid.
>
> In case if a raid1, if a disk fails you get a message from the system.
> If you have spare disks (configured and installed as so in the raid),
> the raid is rebuilt on the spare disk. You will notice disk activity
> related to this mirroring. Then you can wait until you can shutdown
> your system and remove/replace the defective disk. Then restart the
> system and use mdadm to reinstall the disk in the array.
> If you have no spare, the raid is degraded and you are running on the
> safe disk without any redundancy.
> You have, the same way as before, to shutdown your system and remove /
> replace the defective disk.
>
> In case of a raid0, you have no redundancy and the filesystem relying
> on this raid will die.
>
> Remarks :
> - SATA is told to be hotplug but most of the motherboards dont support
> hotplugging of disks on their SATA controller. This is why you have to
> shutdown your system.
Actually most SATA controllers DO support hotplug. The linux kernel
doesn't support SATA hot plug yet (although supposedly that is being
worked on).
> - There are disk failures and there are controller failures. If both
> your disk are on the same controller, your system will crash.
If your controller fails, most likely the system will crash when the
driver gets very unexpected results. This is a rather unusual case, and
very expensive to protect against. Disk failures are much more common
and fortunately much easier to protect against.
> - The swap has to be on the raid also (or on a logical volume of LVM
> which is built over the raid) otherwise, you will probably crash your
> system at the failure.
That is for sure.
> You can download and install mdadm : the doc files in
> /usr/share/doc/mdadm contain valuable informations.
>
> It is often easier to repair have access to ext3 (which is
> ext2+journal) from a system you have booted from a live CD, just in
> case of a weird problem on the filesystem.
Also grub only supports some filesystems for /boot, so it makes sense to
keep / as something supported and simple. After all if your / is small
and doesn't change much why would you need any fancy filesystem for it?
--
Len Sorensen
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