Re: [gentoo-user] Is an Intel motherboard RAID better or worse than software RAID?

[Volker Armin Hemmann <volker.armin.hemmann@tu-clausthal.de>]

On Montag 20 Oktober 2008, Conway S. Smith wrote:
> On Mon, 20 Oct 2008 08:54:20 +0200
>
> Wolfgang Liebich <Wolfgang.Liebich@siemens.com> wrote:
> > Hi,
> >
> > <SNIP>
> >
> > > 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. 

just don't use an initramfs/initrd.

> > 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
> partitions).

also, reiserfs and xfs turn barriers on by default, ext3 turns it off per 
default. Because of 'performance reasons'.