Raid1 DM vs MD

Raid1 DM vs MD

[Kianusch Sayah Karadji]

Hi!

After loosing some Data this week ... the question upon with technology to 
use for Soft-RAID1 emerged the last days.

So which one is the recomended approach?

Should I use MD or DM?

One benefit on using MD is that one can use it for root-devices without 
initrd.

But where will the development go?

Will MD be supported in the future or will it be replaced by DM?

Will there be other raid levels supported in DM?

Which one has better Clean/Dirty recognition/detection?

I had one MD-Raid1 where a good copy of the mirror was overwritten by the 
bad (old) copy ... I lost 3 Month worth of data and I am expecting loosing 
a linux project and in the worst case - even a court case :(

Questions upon questions.

Sooner or later I'l migrate from SW-Raid to a HW-Raid-Controller ...

Thanx
Kianusch

---
   SK-TECH.net
   http://www.sk-tech.net

Re: Raid1 DM vs MD

[Jan Engelhardt]

>I had one MD-Raid1 where a good copy of the mirror was overwritten by the
>bad (old) copy ... I lost 3 Month worth of data and I am expecting loosing
>a linux project and in the worst case - even a court case :(

Yep, happened to me too, and ever since, I refrain from using MD, but instead a
good (read: my own) backup solution. It's even a little cheaper since you do
not to spare a partition/harddisk of the same size you're mirroring, but can
simply put the .tar.bz2 (+.acl.bz2) onto another fs.



Jan Engelhardt
-- 
Gesellschaft für Wissenschaftliche Datenverarbeitung
Am Fassberg, 37077 Göttingen, www.gwdg.de

Re: Raid1 DM vs MD

[Bernd Eckenfels]

In article <Pine.LNX.4.61.0410311902300.1819@merlin.sk-tech.net> you wrote:
> I had one MD-Raid1 where a good copy of the mirror was overwritten by the 
> bad (old) copy ... I lost 3 Month worth of data and I am expecting loosing 
> a linux project and in the worst case - even a court case :(

This is the expected behaviour in  your situation.

I mean, if you dont do backup for month something has to go wrong :)

But more seriouly, when did that overwrite happen? After reboot or have you
changed media? Was the data lost because of corruption or because the old
copy did not contained the data?

Bernd

Re: Raid1 DM vs MD

[Neil Brown]

On Sunday October 31, kianusch@sk-tech.net wrote:
> Hi!
> 
> After loosing some Data this week ... the question upon with technology to 
> use for Soft-RAID1 emerged the last days.
> 
> So which one is the recomended approach?
> 
> Should I use MD or DM?

I would say "md", but I am biased.

> 
> One benefit on using MD is that one can use it for root-devices without 
> initrd.
> 
> But where will the development go?

Forwards.
It is unlikely that support for useful functionality will go away.
I would prefer to see all arrays being assembled by an initrd-like
thing (initramfs??) but wouldn't dream of encouraging that until it
was easy to use and widely used.  We aren't there yet.

> 
> Will MD be supported in the future or will it be replaced by DM?

It is unlikely that md will be replaced by dm in the foreseeable
future.
It is possible that dm will increase in functionality and become very
widely used.

> 
> Will there be other raid levels supported in DM?
> 
> Which one has better Clean/Dirty recognition/detection?

While I know no details of DM, I doubt there is substantial difference
here.  It isn't a hard problem.

> 
> I had one MD-Raid1 where a good copy of the mirror was overwritten by the 
> bad (old) copy ... I lost 3 Month worth of data and I am expecting loosing 
> a linux project and in the worst case - even a court case :(

That sounds very unfortunate.  Without knowing the details it is hard
to comment on why this might have happened and how it could have been
avoided, but with modern tools (mdadm) and a sufficiently modern
kernel (2.4 at least) this should never be able to happen (without
deliberate carelessness on the part of the sysadmin).


> 
> Questions upon questions.
> 
> Sooner or later I'l migrate from SW-Raid to a HW-Raid-Controller ...

Many believe that that would not be a win.  Personally I share that
view.  HW-Raid-Controllers are not "open".  md (and dm) SW-Raid is.

Good Luck.

NeilBrown

Re: Raid1 DM vs MD

[Kianusch Sayah Karadji]

Hi!

>> Should I use MD or DM?
>
> I would say "md", but I am biased.

:)

> It is unlikely that support for useful functionality will go away. I 
> would prefer to see all arrays being assembled by an initrd-like thing 
> (initramfs??) but wouldn't dream of encouraging that until it was easy 
> to use and widely used.  We aren't there yet.

It seem that there has been some changes lately on the way when and by 
whom the RAID devices are created (especially in the 2.6.x stream) ... but 
one of the most frustrating experiences is the lack of decent 
documentation - or maybe some examples... Raid Support on DM seems not be 
documented at all ... and than there are some "SubTypes" like Raid 
Semi-Raid-Controllers (CMD649 etc.).  Or maybe I'm searched the wrong way 
on google?  Is there a central resource on the net for Linux & Raid I do 
not know of?

>> I had one MD-Raid1 where a good copy of the mirror was overwritten by 
>> the bad (old) copy ... I lost 3 Month worth of data and I am expecting 
>> loosing a linux project and in the worst case - even a court case :(
>
> That sounds very unfortunate.  Without knowing the details it is hard to 
> comment on why this might have happened and how it could have been 
> avoided, but with modern tools (mdadm) and a sufficiently modern kernel 
> (2.4 at least) this should never be able to happen (without deliberate 
> carelessness on the part of the sysadmin).

Hmmm ...

RedHat ES 2.x ... so no mdadm but raidtools ... The System was set up with 
HA-Linux so the sysadmin just had to initiate the TakeOver ... According 
to /dev/mdstat the System was okay ... but where do I see which copy is 
the "master" and where do I see in which direction the synchronisation is 
going to take place - before the raid is started?

Also I did not find a way to start the raid with just one of the mirrors.

As I said my biggest problem is the lack of documentation.

>> Sooner or later I'l migrate from SW-Raid to a HW-Raid-Controller ...
>
> Many believe that that would not be a win.  Personally I share that 
> view.  HW-Raid-Controllers are not "open".  md (and dm) SW-Raid is.

Well ... yes ... but with HW-Raid users do not have to think very much. 
(I'm talking about REAL Raid Controllers - like 3ware, ICP, Adaptek - not 
about Soft-Raid-Controllers) Simply plug in the HardDisk und look how the 
system syncs the disc.  And in my case I'm talking about mirroring 2TB 
Data ... MD takes up to 12Hours for syncing the raid - and 
stopping/starting MD during this time is always risky.

Kianusch

Re: Raid1 DM vs MD

[Bernd Eckenfels]

In article <Pine.LNX.4.61.0411011141001.1412@merlin.sk-tech.net> you wrote:
> Well ... yes ... but with HW-Raid users do not have to think very much. 

Actually they have the same problems. And with a Failover Cluster, you need
special Hardware (and most often special drivers which are not supported on
Linux)

Greetings
Bernd

Re: Raid1 DM vs MD

[Måns Rullgård]

Neil Brown <neilb@cse.unsw.edu.au> writes:

>> Sooner or later I'l migrate from SW-Raid to a HW-Raid-Controller ...
>
> Many believe that that would not be a win.  Personally I share that
> view.  HW-Raid-Controllers are not "open".  md (and dm) SW-Raid is.

What happened to the proposed standard for on-disk format?  Didn't
Adaptec also publish their format?

-- 
Måns Rullgård
mru@inprovide.com