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MDADM creates additional block devices

mdadm - RAID5 array size vs. actual disk size mismatch“Spare” disk in a 2-disk mdadm RAID1 array?An existing mdadm RAID5 is not mounting, Either a problem drive or SuperblockReplacing a disk to RAID5 failedraid5 array reassembles as raid0How do I (re)build/create/assemble an IMSM RAID-0 array from disk images instead of disk drives using mdadm?Recovering a RAID 6mdadm raid10 recovery - is this filesystem corrupted? Is it fixable?

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0

I noticed a strange issue when creating RAID array with mdadm. As far as I see, it creates additional devices of md type. According to the kernel.org documentation, the md device type is Metadisk (RAID) devices.

The metadisk driver is used to span a filesystem across multiple
physical disks.

However, this started to happens today on the same host. (I am using a configuration management tool to create a RAID array, but the same happens when I issue the mdadm commands manually.

lsblk output looks like below:

user@host:~$ lsblk

NAME      MAJ:MIN RM  SIZE RO TYPE  MOUNTPOINT

nvme0n1   259:0    0  1.7T  0 disk

└─md0       9:0    0  3.5T  0 raid0

  └─md0p2 259:2    0  1.8T  0 md

xvda      202:0    0   20G  0 disk

└─xvda1   202:1    0   20G  0 part  /

xvdf      202:80   0   20G  0 disk

nvme1n1   259:1    0  1.7T  0 disk

└─md0       9:0    0  3.5T  0 raid0

  └─md0p2 259:2    0  1.8T  0 md

Can someone shed some light? Is this expected? I am seeing this for the first time.

Host is Ubuntu 16.04.6 LTS

Clarification

Why are these created:

 └─md0p2 259:2    0  1.8T  0 md

linux devices mdadm lsblk raid0

share|improve this question

edited 16 hours ago

Alan Kis

asked 16 hours ago

Alan KisAlan Kis

16622 gold badges33 silver badges99 bronze badges

• 
  
  
  

  
  

  
  
  
  
  
  

  
  
  What exactly is the problem you're seeing in lsblk output?
  
  – muru
  16 hours ago
  

  
  
  
  


• 
  
  
  

  
  1
  

  
  
  
  
  
  

  
  
  Do you mean the md0p2? That's just a partition that somebody created on the md0 device, just like you would on any other disk device.
  
  – TooTea
  16 hours ago
  

  
  
  
  


• 
  
  
  

  
  

  
  
  
  
  
  

  
  
  @muru the md0p2 devices that are created along.
  
  – Alan Kis
  16 hours ago
  

  
  
  
  


• 
  
  
  

  
  

  
  
  
  
  
  

  
  
  @TooTea md0p2 is not a type PARTITION. It is MD type. No one except me and the NSA doesn't have access to the host.
  
  – Alan Kis
  16 hours ago
  

  
  
  
  


• 
  
  
  

  
  

  
  
  
  
  
  

  
  
  It's still a partition. If you didn't create it yourself, there might have been a partition table lying around on one of the component devices and that partition table is now seen through the raid device. That sounds likely given that the size of the partition looks very close to the size of the component device.
  
  – TooTea
  15 hours ago
  

  
  
  
  

 | 
show 2 more comments

0

I noticed a strange issue when creating RAID array with mdadm. As far as I see, it creates additional devices of md type. According to the kernel.org documentation, the md device type is Metadisk (RAID) devices.

The metadisk driver is used to span a filesystem across multiple
physical disks.

However, this started to happens today on the same host. (I am using a configuration management tool to create a RAID array, but the same happens when I issue the mdadm commands manually.

lsblk output looks like below:

user@host:~$ lsblk

NAME      MAJ:MIN RM  SIZE RO TYPE  MOUNTPOINT

nvme0n1   259:0    0  1.7T  0 disk

└─md0       9:0    0  3.5T  0 raid0

  └─md0p2 259:2    0  1.8T  0 md

xvda      202:0    0   20G  0 disk

└─xvda1   202:1    0   20G  0 part  /

xvdf      202:80   0   20G  0 disk

nvme1n1   259:1    0  1.7T  0 disk

└─md0       9:0    0  3.5T  0 raid0

  └─md0p2 259:2    0  1.8T  0 md

Can someone shed some light? Is this expected? I am seeing this for the first time.

Host is Ubuntu 16.04.6 LTS

Clarification

Why are these created:

 └─md0p2 259:2    0  1.8T  0 md

linux devices mdadm lsblk raid0

share|improve this question

edited 16 hours ago

Alan Kis

asked 16 hours ago

Alan KisAlan Kis

16622 gold badges33 silver badges99 bronze badges

• 
  
  
  

  
  

  
  
  
  
  
  

  
  
  What exactly is the problem you're seeing in lsblk output?
  
  – muru
  16 hours ago
  

  
  
  
  


• 
  
  
  

  
  1
  

  
  
  
  
  
  

  
  
  Do you mean the md0p2? That's just a partition that somebody created on the md0 device, just like you would on any other disk device.
  
  – TooTea
  16 hours ago
  

  
  
  
  


• 
  
  
  

  
  

  
  
  
  
  
  

  
  
  @muru the md0p2 devices that are created along.
  
  – Alan Kis
  16 hours ago
  

  
  
  
  


• 
  
  
  

  
  

  
  
  
  
  
  

  
  
  @TooTea md0p2 is not a type PARTITION. It is MD type. No one except me and the NSA doesn't have access to the host.
  
  – Alan Kis
  16 hours ago
  

  
  
  
  


• 
  
  
  

  
  

  
  
  
  
  
  

  
  
  It's still a partition. If you didn't create it yourself, there might have been a partition table lying around on one of the component devices and that partition table is now seen through the raid device. That sounds likely given that the size of the partition looks very close to the size of the component device.
  
  – TooTea
  15 hours ago
  

  
  
  
  

 | 
show 2 more comments

I noticed a strange issue when creating RAID array with mdadm. As far as I see, it creates additional devices of md type. According to the kernel.org documentation, the md device type is Metadisk (RAID) devices.

The metadisk driver is used to span a filesystem across multiple
physical disks.

However, this started to happens today on the same host. (I am using a configuration management tool to create a RAID array, but the same happens when I issue the mdadm commands manually.

lsblk output looks like below:

user@host:~$ lsblk

NAME      MAJ:MIN RM  SIZE RO TYPE  MOUNTPOINT

nvme0n1   259:0    0  1.7T  0 disk

└─md0       9:0    0  3.5T  0 raid0

  └─md0p2 259:2    0  1.8T  0 md

xvda      202:0    0   20G  0 disk

└─xvda1   202:1    0   20G  0 part  /

xvdf      202:80   0   20G  0 disk

nvme1n1   259:1    0  1.7T  0 disk

└─md0       9:0    0  3.5T  0 raid0

  └─md0p2 259:2    0  1.8T  0 md

Can someone shed some light? Is this expected? I am seeing this for the first time.

Host is Ubuntu 16.04.6 LTS

Clarification

Why are these created:

 └─md0p2 259:2    0  1.8T  0 md

linux devices mdadm lsblk raid0

share|improve this question

edited 16 hours ago

Alan Kis

asked 16 hours ago

Alan KisAlan Kis

16622 gold badges33 silver badges99 bronze badges

user@host:~$ lsblk

NAME      MAJ:MIN RM  SIZE RO TYPE  MOUNTPOINT

nvme0n1   259:0    0  1.7T  0 disk

└─md0       9:0    0  3.5T  0 raid0

  └─md0p2 259:2    0  1.8T  0 md

xvda      202:0    0   20G  0 disk

└─xvda1   202:1    0   20G  0 part  /

xvdf      202:80   0   20G  0 disk

nvme1n1   259:1    0  1.7T  0 disk

└─md0       9:0    0  3.5T  0 raid0

  └─md0p2 259:2    0  1.8T  0 md

 └─md0p2 259:2    0  1.8T  0 md

share|improve this question

edited 16 hours ago

Alan Kis

asked 16 hours ago

Alan KisAlan Kis

16622 gold badges33 silver badges99 bronze badges

share|improve this question

edited 16 hours ago

Alan Kis

asked 16 hours ago

Alan KisAlan Kis

16622 gold badges33 silver badges99 bronze badges

asked 16 hours ago

Alan KisAlan Kis

16622 gold badges33 silver badges99 bronze badges

asked 16 hours ago

Alan KisAlan Kis

16622 gold badges33 silver badges99 bronze badges

1 Answer
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1

When a block device is added to the system, the kernel attempts to parse any partition table that may exist on the device, and if successful, will add block devices for the partitions it thinks exists because of what it read in the partition table. Note that support for many partition table types can be configured into the kernel, so partition tables from e.g. Solaris and BSD will also be recognized.

It's possible for random data to somehow match what the kernel expects a partition table to look like, causing false partition devices to be created.

lsblk output has a "TYPE" column, but that may or may not be correct; this too may be a best-effort guess. The major device number is a better indication of what type of device it is; refer to /proc/devices to see what numbers correspond to what kernel driver. Here at least the md0p2 major device number is not the same as the md0 major device number, so it's not the md driver that created that device.

Zeroing out the first block helps prevent such misinterpretation of data as a partition table:

# dd if=/dev/zero of=/dev/md0 count=1

Double-check the device you are writing to!

share|improve this answer

answered 13 hours ago

wurtelwurtel

11.8k11 gold badge1616 silver badges2929 bronze badges

add a comment | 

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1

When a block device is added to the system, the kernel attempts to parse any partition table that may exist on the device, and if successful, will add block devices for the partitions it thinks exists because of what it read in the partition table. Note that support for many partition table types can be configured into the kernel, so partition tables from e.g. Solaris and BSD will also be recognized.

It's possible for random data to somehow match what the kernel expects a partition table to look like, causing false partition devices to be created.

lsblk output has a "TYPE" column, but that may or may not be correct; this too may be a best-effort guess. The major device number is a better indication of what type of device it is; refer to /proc/devices to see what numbers correspond to what kernel driver. Here at least the md0p2 major device number is not the same as the md0 major device number, so it's not the md driver that created that device.

Zeroing out the first block helps prevent such misinterpretation of data as a partition table:

# dd if=/dev/zero of=/dev/md0 count=1

Double-check the device you are writing to!

share|improve this answer

answered 13 hours ago

wurtelwurtel

11.8k11 gold badge1616 silver badges2929 bronze badges

add a comment | 

1

When a block device is added to the system, the kernel attempts to parse any partition table that may exist on the device, and if successful, will add block devices for the partitions it thinks exists because of what it read in the partition table. Note that support for many partition table types can be configured into the kernel, so partition tables from e.g. Solaris and BSD will also be recognized.

It's possible for random data to somehow match what the kernel expects a partition table to look like, causing false partition devices to be created.

lsblk output has a "TYPE" column, but that may or may not be correct; this too may be a best-effort guess. The major device number is a better indication of what type of device it is; refer to /proc/devices to see what numbers correspond to what kernel driver. Here at least the md0p2 major device number is not the same as the md0 major device number, so it's not the md driver that created that device.

Zeroing out the first block helps prevent such misinterpretation of data as a partition table:

# dd if=/dev/zero of=/dev/md0 count=1

Double-check the device you are writing to!

share|improve this answer

answered 13 hours ago

wurtelwurtel

11.8k11 gold badge1616 silver badges2929 bronze badges

add a comment | 

When a block device is added to the system, the kernel attempts to parse any partition table that may exist on the device, and if successful, will add block devices for the partitions it thinks exists because of what it read in the partition table. Note that support for many partition table types can be configured into the kernel, so partition tables from e.g. Solaris and BSD will also be recognized.

It's possible for random data to somehow match what the kernel expects a partition table to look like, causing false partition devices to be created.

lsblk output has a "TYPE" column, but that may or may not be correct; this too may be a best-effort guess. The major device number is a better indication of what type of device it is; refer to /proc/devices to see what numbers correspond to what kernel driver. Here at least the md0p2 major device number is not the same as the md0 major device number, so it's not the md driver that created that device.

Zeroing out the first block helps prevent such misinterpretation of data as a partition table:

# dd if=/dev/zero of=/dev/md0 count=1

Double-check the device you are writing to!

share|improve this answer

answered 13 hours ago

wurtelwurtel

11.8k11 gold badge1616 silver badges2929 bronze badges

# dd if=/dev/zero of=/dev/md0 count=1

share|improve this answer

answered 13 hours ago

wurtelwurtel

11.8k11 gold badge1616 silver badges2929 bronze badges

answered 13 hours ago

wurtelwurtel

11.8k11 gold badge1616 silver badges2929 bronze badges

answered 13 hours ago

wurtelwurtel

11.8k11 gold badge1616 silver badges2929 bronze badges