On Mon, 20 Oct 2008 08:54:20 +0200
Wolfgang Liebich <Wolfgang.Liebich@siemens.com> wrote:
> > the howtos on gentoo-wiki worked well for me.
> I'm working with them, too. Just one question remains: I want to use
> udev. Do I have to create the md devices or does udev that for me?
udev will do it for you. But make sure your initramfs init script
unmounts /sys & /proc. On the box I'm working on setting up it
wasn't unmounting /sys on the initramfs, so when it switched to the
real root it thought /sys was already mounted & didn't mount /sys
under the real root, which meant that udev didn't work - which took
me a while to figure out.
> > > - Put the root partition on another RAID1 (I thought about
> > > putting the root filesystem into my LVM setup, too -- it is
> > > REALLY annoying if the root partition get's to small),
> > yeah, but if you have 20+ gb root is always big enough
> > lvm kills barriers. You use raid for better data security. So
> > using lvm is a bit.. contra productive.
> Sorry, I'm neither a LVM nor a RAID export - could you please
> elaborate on that?
> I like LVM because of the convenience it adds.
Write barriers are a feature to allow write caching on the hard disks
w/out endangering filesystem integrity. Write caching helps
performance significantly, but also allows the disk to re-order write
requests - the disk may actually write a write-request that was
received later before a write-request that was received earlier,
which in some situations can lead to filesystem corruption. Write
barriers are a special type of request that the disk is not allowed
to reorder around - everything the disk receives before the write
barrier must be written before anything received after the write
barrier. But in order to work, write barriers need to be supported
by every layer from the filesystem down to the actual disk; if your
filesystem is on top of LVM & LVM doesn't support write barriers,
then you won't be able to use them, and if write caching is enabled
on the actual disks, you may be risking fileystem corruption. The
Device Mapper kernel subsystem (dm-crypt, dm-raid, LVM, etc.) does
not support write barriers - but neither does MD RAID except for
RAID1, so write caching is dangerous except for filesystems directly
on disk partitions or on RAID1 (if the RAID1 is directly on disk
I personally decided against using LVM because from what I read it's
difficult to correctly stripe-align LVM, and incorrect alignment can
have a very big performance impact.
Conway S. Smith
The only "intuitive" interface is the nipple. After that, it's all
learned. (Bruce Ediger, firstname.lastname@example.org, in comp.os.linux.misc,
on X interfaces.)