Comments on: LVM2 Mirrors vs. MD Raid 1

By: ???????? ? mdadm ? lvm | Just another IT blog…

???????? ? mdadm ? lvm | Just another IT blog… — Tue, 18 Oct 2011 08:12:33 +0000

[...] ????? ????????? ?????????? ????? ????? ? ????????? (????????: http://serverfault.com/questions/126851/linux-lvm-mirror-vs-md-mirror ??? http://www.joshbryan.com/blog/2008/01/02/lvm2-mirrors-vs-md-raid-1/). [...]

By: LVM mirroring VS RAID1 - Admins Goodies

LVM mirroring VS RAID1 - Admins Goodies — Sun, 21 Aug 2011 08:24:05 +0000

[...] http://www.joshbryan.com/blog/2008/01/02/lvm2-mirrors-vs-md-raid-1/ [...]

By: gwpl

gwpl — Mon, 25 Jul 2011 11:35:39 +0000

Update: write barriers are fully supported in linux kernel 2.6.33 according to : http://en.wikipedia.org/wiki/Logical_Volume_Manager_%28Linux%29

By: Linux LVM mirroring comes at a price | deranfangvomen.de

Linux LVM mirroring comes at a price | deranfangvomen.de — Mon, 04 Apr 2011 23:31:59 +0000

[...] You can find some nice article about clvm mirroring here http://www.joshbryan.com/blog/2008/01/02/lvm2-mirrors-vs-md-raid-1 [...]

By: darkfader

darkfader — Mon, 04 Apr 2011 23:09:05 +0000

Definitely follow the advice from post #3

Barriers are lost in devicemapper layer, so unless you use a filesystem right on top of the md (or DR:BD) devices you are in this situation.

[ ] Faster
[ ] Safe
chose one.

I’ll add some moreo n that in my blog and link back here

By: Der neue Server: Teil 1 Installation und Basiskonfiguration | johker’s blog

Tue, 17 Mar 2009 19:32:05 +0000

[...] Der Grund ist einfach: über ein Software-RAID erhalte ich höhere Leseraten (siehe auch: http://www.joshbryan.com/blog/2008/01/02/lvm2-mirrors-vs-md-raid-1/, insbesondere auch die Kommentare). Beim Dateisystem setze ich auf ext3 bzw. ext2 für [...]

By: Prateek Donni

Prateek Donni — Wed, 21 Jan 2009 06:06:32 +0000

How does RAID behave in case of presence of an intermittently connected storage, and in case of hot unplug of the same on a booted system ?

By: Scott C.

Scott C. — Thu, 02 Oct 2008 21:30:08 +0000

LVM is not safe in a power failure, it does not respect write barriers and pass those down to the lower drives.

hence, it is often faster than MD by default, but to be safe you would have to turn off your drive’s write caches, which ends up making it slower than if you used write barriers.

http://lwn.net/Articles/283161/

“Unless, of course, their filesystem is based on an LVM volume (as certain distributions do by default); it turns out that the device mapper code does not pass through or honor barriers.”

By: Markus

Markus — Wed, 02 Jul 2008 07:17:05 +0000

Hi,
I also made some comparisons, but under Ubuntu Hardy.
My first results showed almost the same throughput for LVM-mirror and MD-RAID1 with a slight better performance on LVM-mirror (but only 1-2 MB/sec).
But then I tested with _two_ files simultaneously on the same filesystem and there I found a much better performance on read with MD-RAID1 (about 1.5 times of LVM-mirror or no mirror).
The explanation is: MD always reads simultaneously from both disks, whereas LVM-mirror only reads from one mirror and uses the second mirror only in case of failure!
That’s a poor design flaw in my opinion.

This design flaw and the fact, that on my system mirror failures were not recognized correctly, makes LVM-mirror useless for me. I suspect one needs dmeventd to recognize mirror failures correctly, but this is not yet available in Ubuntu and I don’t want to go into long compilation sessions.

It’s a pity, because I like the idea of LVM-mirroring more than the MD-mirroring.

By: JP

JP — Mon, 12 May 2008 16:25:22 +0000

Thanks for the comparison. I too am having the same debate. If LVM is just as fast or faster, there seems to be no question which is the better solution. Have you noticed any issues since this post?