LVM, MDADM, and MD RAID

Obsolesce

@black3dynamite said in Miscellaneous Tech News:

@obsolesce said in Miscellaneous Tech News:

@scottalanmiller said in Miscellaneous Tech News:

@obsolesce said in Miscellaneous Tech News:

@black3dynamite said in Miscellaneous Tech News:

@dustinb3403 said in Miscellaneous Tech News:

It appears the XCP-ng team is working on better software raid integration.

ISO

That's cool. We can have LVM on top of software raid.

LVM does Raid.

LVM only provides an interface to the RAID, it doesn't do the RAID. Just like MDADM is an interface to MD, LVM is an interface to MD. But it is MD in both cases.

So lvcreate --type raid5 ...etc just runs some mdadm commands behind the scenes?

So it creates the raid using mdadm and then the logical volume?

I'm asking, I didn't think that was the case, but @scottalanmiller says it is. I'm trying to find this info. I'm reading more about LVM now to see exactly what it does, but I haven't found anything relating to MDADM at all.

https://www.centos.org/docs/5/html/Cluster_Logical_Volume_Manager/lv_overview.html

Obsolesce

@scottalanmiller
Can this be forked from LVM / MDADM? I want to discuss this in it's own topic.

Obsolesce

I think the confusion is this:

You can use LVM on MD, or straight LVM. It seems LVM only was buggy in the past, but is good now.

Obsolesce

Just found this:
https://mangolassi.it/topic/493/linux-lvm-and-mdadm-merging-md-raid-handling-code

But is old. So is LVM still actually using MD behind the scenes? I really can't find anything official on this anywhere.

scottalanmiller

@black3dynamite said in LVM, MDADM, and MD RAID:

@obsolesce said in Miscellaneous Tech News:

@scottalanmiller said in Miscellaneous Tech News:

@obsolesce said in Miscellaneous Tech News:

@black3dynamite said in Miscellaneous Tech News:

@dustinb3403 said in Miscellaneous Tech News:

It appears the XCP-ng team is working on better software raid integration.

ISO

That's cool. We can have LVM on top of software raid.

LVM does Raid.

LVM only provides an interface to the RAID, it doesn't do the RAID. Just like MDADM is an interface to MD, LVM is an interface to MD. But it is MD in both cases.

So lvcreate --type raid5 ...etc just runs some mdadm commands behind the scenes?

So it creates the raid using mdadm and then the logical volume?

No, LVM and MDADM compete. They BOTH are interfaces to MD. LVM uses MD, not MDADM.

scottalanmiller

@obsolesce said in LVM, MDADM, and MD RAID:

Just found this:
https://mangolassi.it/topic/493/linux-lvm-and-mdadm-merging-md-raid-handling-code

But is old. So is LVM still actually using MD behind the scenes? I really can't find anything official on this anywhere.

Might feel old, but that's not very old and nothing has changed. And why would it change? Doesn't make sense. Merged makes sense.

scottalanmiller

@obsolesce said in LVM, MDADM, and MD RAID:

I think the confusion is this:

You can use LVM on MD, or straight LVM. It seems LVM only was buggy in the past, but is good now.

LVM was buggy in what way?

scottalanmiller

@obsolesce said in LVM, MDADM, and MD RAID:

@black3dynamite said in Miscellaneous Tech News:

@obsolesce said in Miscellaneous Tech News:

@scottalanmiller said in Miscellaneous Tech News:

@obsolesce said in Miscellaneous Tech News:

@black3dynamite said in Miscellaneous Tech News:

@dustinb3403 said in Miscellaneous Tech News:

It appears the XCP-ng team is working on better software raid integration.

ISO

That's cool. We can have LVM on top of software raid.

LVM does Raid.

LVM only provides an interface to the RAID, it doesn't do the RAID. Just like MDADM is an interface to MD, LVM is an interface to MD. But it is MD in both cases.

So lvcreate --type raid5 ...etc just runs some mdadm commands behind the scenes?

So it creates the raid using mdadm and then the logical volume?

I'm asking, I didn't think that was the case, but @scottalanmiller says it is. I'm trying to find this info. I'm reading more about LVM now to see exactly what it does, but I haven't found anything relating to MDADM at all.

https://www.centos.org/docs/5/html/Cluster_Logical_Volume_Manager/lv_overview.html

Two reasons for this...

It's related to MD, not MDADM.
Your docs are for version 5 which predates the merger around 6.2.

scottalanmiller

Here is another article. Of course they mistakenly call MD MDADM as so many people do. BUt the ideas are what matter.

https://unix.stackexchange.com/questions/150644/raiding-with-lvm-vs-mdraid-pros-and-cons#182503

Emad R

@obsolesce

Just putting this here cause I like it and stole it from:
https://serverfault.com/questions/217666/what-is-better-lvm-on-raid-or-raid-on-lvm
and helped me recently

| / | /var | /usr | /home  |
 --------------------------
|       LVM Volume         |
 --------------------------
|       RAID Volume        |
 --------------------------
| Disk 1 | Disk 2 | Disk 3 |

Obsolesce

I see...

So MD is the multiple device driver / RAID implementation built into the Linux kernel. Both LVM and MDADM can interface to MD, LVM being more featurefull than MDADM?

travisdh1

@emad-r said in LVM, MDADM, and MD RAID:

@obsolesce

Just putting this here cause I like it and stole it from:
https://serverfault.com/questions/217666/what-is-better-lvm-on-raid-or-raid-on-lvm
and helped me recently
| / | /var | /usr | /home  |
 --------------------------
|       LVM Volume         |
 --------------------------
|       RAID Volume        |
 --------------------------
| Disk 1 | Disk 2 | Disk 3 |

I don't like them calling it LVM Volume as that's something very specific within LVM. Can't tell you how long this sort of wording confused the heck out of me.

matteo nunziati

@obsolesce mdadm predates lvm. Mdadm was about raid, lvm about thick/thin provisioned volumes which were like on-the-fly resizeable partitions. Usually you see mdadm -more battle tested- for raid management and lvm for flexible partitioning on top of mdadm.
Later they added raid capabilities to lvm. But honestly I ignored this!

scottalanmiller

@obsolesce said in LVM, MDADM, and MD RAID:

I see...

So MD is the multiple device driver / RAID implementation built into the Linux kernel. Both LVM and MDADM can interface to MD, LVM being more featurefull than MDADM?

Correct. LVM is more generally featureful, MDADM is slightly more featureful just within the set of RAID features.

scottalanmiller

@travisdh1 said in LVM, MDADM, and MD RAID:

| / | /var | /usr | /home |

| LVM Volume |

| RAID Volume |

| Disk 1 | Disk 2 | Disk 3 |

Right, it's really LVM and RAID, not volumes. Volumes exists within the RAID stack and within the LVM stack. In this diagram, that's the whole stack, not just the volumes.

scottalanmiller

@matteo-nunziati said in LVM, MDADM, and MD RAID:

@obsolesce mdadm predates lvm. Mdadm was about raid, lvm about thick/thin provisioned volumes which were like on-the-fly resizeable partitions. Usually you see mdadm -more battle tested- for raid management and lvm for flexible partitioning on top of mdadm.
Later they added raid capabilities to lvm. But honestly I ignored this!

MD (pre-MDADM) was 2001. LVM on Linux was 1998. So LVM has a 3+ year lead on MD. We were using LVM heavily when MD came around.

My time on Linux predates both.

matteo nunziati

@scottalanmiller woa! I was sure it was the opposite! I really don't know where I learned it... Anyway my time w/ linux begins in 2004/2005 and I've always been more on the coding side of things. Interesting discovery!

scottalanmiller

LVM was basically a copy of the LVM on HP-UX.

travisdh1

@scottalanmiller said in LVM, MDADM, and MD RAID:

LVM was basically a copy of the LVM on HP-UX.

SGI people claimed that MD was a copy of their software based RAID back in the late 90s. How true that was, I really don't know.

scottalanmiller

@travisdh1 said in LVM, MDADM, and MD RAID:

@scottalanmiller said in LVM, MDADM, and MD RAID:

LVM was basically a copy of the LVM on HP-UX.

SGI people claimed that MD was a copy of their software based RAID back in the late 90s. How true that was, I really don't know.

All early software RAID is pretty similar.