LVM, MDADM, and MD RAID
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MD is a device driver so it assembles several devices into one.
For instance /dev/sda and /dev/sdb into /dev/md0.
It has no clue about what kind of file system or anything like that it's running. It works on the block level, just like hardware raid.
You would not get a volume until you have a partition somewhere that you format with a file system.
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@pete-s said in LVM, MDADM, and MD RAID:
MD is a device driver so it assembles several devices into one.
For instance /dev/sda and /dev/sdb into /dev/md0.
It has no clue about what kind of file system or anything like that it's running. It works on the block level, just like hardware raid.
You would not get a volume until you have a partition somewhere that you format with a file system.
The concept of Drive Appearance is what is at play in both RAID and volume managers.
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@scottalanmiller said in LVM, MDADM, and MD RAID:
@pete-s said in LVM, MDADM, and MD RAID:
MD is a device driver so it assembles several devices into one.
For instance /dev/sda and /dev/sdb into /dev/md0.
It has no clue about what kind of file system or anything like that it's running. It works on the block level, just like hardware raid.
You would not get a volume until you have a partition somewhere that you format with a file system.
The concept of Drive Appearance is what is at play in both RAID and volume managers.
I always thought that RAID and LVM were two different concepts... RAID for Data protection (ignoring RAID 0) and LVM for data presentation... ?
Am I close in that way of thinking or way off base?
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@dafyre said in LVM, MDADM, and MD RAID:
@scottalanmiller said in LVM, MDADM, and MD RAID:
@pete-s said in LVM, MDADM, and MD RAID:
MD is a device driver so it assembles several devices into one.
For instance /dev/sda and /dev/sdb into /dev/md0.
It has no clue about what kind of file system or anything like that it's running. It works on the block level, just like hardware raid.
You would not get a volume until you have a partition somewhere that you format with a file system.
The concept of Drive Appearance is what is at play in both RAID and volume managers.
I always thought that RAID and LVM were two different concepts... RAID for Data protection (ignoring RAID 0) and LVM for data presentation... ?
Am I close in that way of thinking or way off base?
You are correct. Logical volume management (lower case) is a general concept around storage abstraction. RAID is a specific storage virtualization technology that combines multiple drive (appearances) into one in specific ways. Both are storage abstraction and virtualization, but they are different in what they do with that.
Now Linux' LVM (upper case, product name not product category) does some non logical volume management features, like talking to the MD RAID system to configure it for you. That's a feature of that specific LVM product, not an LVM concept.
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The comments here insightful, especially about the lvm mirroring.
https://serverfault.com/questions/126851/linux-lvm-mirror-vs-md-mirror -
@momurda said in LVM, MDADM, and MD RAID:
The comments here insightful, especially about the lvm mirroring.
https://serverfault.com/questions/126851/linux-lvm-mirror-vs-md-mirrorThat entire thread is just, bad. So much from people who know just enough to be dangerous.
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Not sure, i think some here are the ones getting this wrong.
LVM mirroring is different than MD RAID 1 from what i can tell, otherwise why arent people using LVM mirroring? There are zero guides on the internet about using LVM to make RAID volumes. -
@momurda said in LVM, MDADM, and MD RAID:
Not sure, i think some here are the ones getting this wrong.
LVM mirroring is different than MD RAID 1 from what i can tell, otherwise why arent people using LVM mirroring? There are zero guides on the internet about using LVM to make RAID volumes.LVM and MDADM are both using the kernel based MD to do the actual RAID functions. They're exactly the same. I have no idea what all those people think is going on, but many of them obviously have no clue.
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I see now. The discussion i linked is from 2010, before lvm acquired this feature
https://unix.stackexchange.com/questions/150644/raiding-with-lvm-vs-mdraid-pros-and-cons
Here is more recent discussion
https://unix.stackexchange.com/questions/150644/raiding-with-lvm-vs-mdraid-pros-and-cons
And here is @scottalanmiller from a while ago
https://community.spiceworks.com/topic/465835-linux-raid-1-or-lvm-mirror -
@travisdh1 said in LVM, MDADM, and MD RAID:
@momurda said in LVM, MDADM, and MD RAID:
Not sure, i think some here are the ones getting this wrong.
LVM mirroring is different than MD RAID 1 from what i can tell, otherwise why arent people using LVM mirroring? There are zero guides on the internet about using LVM to make RAID volumes.LVM and MDADM are both using the kernel based MD to do the actual RAID functions. They're exactly the same. I have no idea what all those people think is going on, but many of them obviously have no clue.
I didn't know it was like that either until this thread.
