On 12/07/2015 20:51, Alex Thorne wrote: > I'm afraid I won't be testing it any time soon -- I don't have any > drives to pair at the moment. As for your comments about dmraid being > 'fake', I'm a little confused. From what you say it sounds like this is > the hardware RAID that comes with many motherboards. Why is hardware > RAID undesirable over software RAID? Presumably the mdraid software > option has an associated performance hit? Is it just that opaque > proprietary firmware can't be trusted with the important task of looking > after our data?
It's because the "RAID" abilities built into most motherboards are really shitty. Very little, if any, optimization going on, no real intelligence, and the whole thing just looks and feels like it's no more than 2 or more volumes shoved into one group. You would probably be better off adding both disks to ove LVM VG and telling the system to do simple striping or mirroring. I always got the feeling that such RAID facilities were little more than a way to get Windows users to have one big disk instead of two smaller ones using the simplest eans possible. Proper hardware RAID using a proper RAID adapter is a whole different story. Those tend to have proper firmware and control the disks properly. They do the optimizations you expect, they will reorder writes and do something sane with reads, and are worth the money is you need RAID. However, most still only do a subset of desirable RAID, usually 0, 1 and 5. Linux software RAID, done in-kernel, is an amazing compromise. It's almost as efficient as good hardware RAID, and covers all bases and RAID types. Yes, it is somewhat more complex and you do have to use management utilities when working with it. It's similarly complex to administrating say LVM (not the same, just same order of magnitude) and comes with the benefit of NOT costing half a server motherboard :-) > > On 6 July 2015 at 16:01, Peter Humphrey <pe...@prh.myzen.co.uk > <mailto:pe...@prh.myzen.co.uk>> wrote: > > On Monday 06 July 2015 10:19:36 Alex Thorne wrote: > > I guess I did mean mdraid, but would you mind explaining the difference > > (I've never used raid so don't know much about this)? Is dmraid > deprecated > > in favour of mdadm? > > Dmraid is the fake RAID that's included on most motherboards these > days; it's > meant for use with Windows and is enabled (or not) in the BIOS. > There are > Linux drivers, but we're always advised to use mdraid instead. > Mdraid is all > in software spread over the kernel, udev and user space*; it's not > influenced > at all by Windows as far as I know. Mdadm is the user-space > administration > program that comes with mdraid. > > Mdadm creates /dev/mdX from one or more /dev/sdX or similar - e.g. > my /dev/md1 > is built on /dev/sd[ab]1; /dev/md5 is on /dev/sd[ab]5 and /dev/md7 is on > /dev/sd[ab]7. That last one also has LVM on it with a dozen or more > logical > volumes for segments of my overall file system. > > If you want to play with mdraid, the old Gentoo guide is succinct > but useful: > > http://wwwold.gentoo.org/doc/en/gentoo-x86+raid+lvm2-quickinstall.xml > > Well, it was, but suddenly it isn't there - even Google's search > results end > up in an empty page. > > Ah, I've found the new version at https://wiki.gentoo.org/wiki/LVM . > It must > be very new - would you like to test it? :-) > > * Yes, I know that udev runs in user space (="User Device" > management) but I > thought it was worth mentioning separately. > > -- > Rgds > Peter > > > -- Alan McKinnon alan.mckin...@gmail.com
