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Old 02-10-2013, 05:36 AM   #1
Registered: Jun 2009
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Question Help understanding how drives/arrays are detected

Hi all,

I've made such a hash of configuring an external RAID enclosure on a RHEL6 box, I've got to the point where I don't know how I actually got to where I am now!

Anyway, I think I have it working now, but I would like some help understanding something.

In my very limited experience of installing disks under linux before, I have just seen them as /dev/sda, /dev/sdb etc, then created partitions which appear as /dev/sda1, /dev/sda2 etc.

Dmesg shows my device as sdc:
[root@db2 backups]# grep sdc /var/log/dmesg
sd 6:0:0:0: [sdc] 3509329920 512-byte logical blocks: (1.79 TB/1.63 TiB)
sd 6:0:0:0: [sdc] Write Protect is off
sd 6:0:0:0: [sdc] Mode Sense: 77 00 10 08
sd 6:0:0:0: [sdc] Write cache: disabled, read cache: enabled, supports DPO and FUA
 sdc: sdc1
sd 6:0:0:0: [sdc] Attached SCSI disk
I created a partition but attempting to format /dev/sdc1 resulted in errors. Eventually, by looking at /proc/partitions, I noticed an entry /dev/dm-0.

Now I understand this is something to do with the drive mapper, but that's all I know!

I have been able to format and use it so I guess it's working okay, but could a kind soul please explain, or speculate, why my device has appeared as /dev/dm-0 and not /dev/sdc1?

Thank you
Old 02-10-2013, 07:24 AM   #2
David the H.
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A raid partition is a virtual device, not a physical one. /dev/sdc points to the physical drive, while /dev/dm-0 is the virtual partition that combines the space of multiple discs.
Old 02-13-2013, 02:46 AM   #3
Registered: Jun 2009
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Hi David the H,

Thank you for your reply, but that doesn't seem to satisfy what I am seeing. Or I am misinterpreting your answer.

My server consists of:
2 x internal HDDs (RAID1) = sda
8 x internal HDDs (RAID10) = sdb
12 x HDDs (external enclosure) (RAID10) = sdc

[root@db2 ~]# df -h
Filesystem            Size  Used Avail Use% Mounted on
/dev/sda3             259G  150G   96G  62% /
tmpfs                  16G     0   16G   0% /dev/shm
/dev/sda1              50M  250K   50M   1% /boot/efi
/dev/sda4              16G  168M   15G   2% /tmpfs
                      1.7T  127G  1.5T   9% /live
/dev/sdb2             184G  163G   12G  94% /newver
/dev/sdb3             184G  105G   70G  61% /test
/dev/sdb4             184G  132G   43G  76% /report
So, why do I see sda1,sda3,sdb2,sdb3 etc but not sdc1? All the above are partitions on various RAIDs.
I accept that sdc is how the physical enclosure is seen by the OS, but why is the partition dm-0 and not sdc1?

Thank you.

Old 02-15-2013, 12:25 PM   #4
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Could you post the entire configuration?
Like output of "fdisk -l", "cat /etc/fstab", raid configuration I guess can be shown by mdadm in some way but it's a long time since I used it so don't really know.


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