[DRBD-user] DRBD and RAID-1

Christian Iversen ci at sikkerhed.org
Mon Nov 30 15:10:45 CET 2009

Note: "permalinks" may not be as permanent as we would like,
direct links of old sources may well be a few messages off.

Hello DRBD-users

We have been using DRBD for nearly a year now, and have enjoyed the many 
benifits of it tremendously.

However, we are trying to set up a new set of DRBD-enabled servers with 
slightly higher performance.

We especially need read performance and higher seek performance, so we 
want to go with a RAID-1 on the primary server. We are aware that DRBD 
can't go faster than the slower of the two servers, but since the 
secondary server only needs to replicate the writes, this is not an 
issue for us, I believe.

The setup we want is the following:

   2 x 1.5TB SATA in RAID-1 (/dev/md1 (sda2, sdb2))

   1 x 1.5TB SATA (/dev/sdb2)

   1 x 1.5TB (/dev/drbd1 from /dev/md1(pri) and /dev/sdb(sec))

Seems to work fine. However, we would like to use LVM on top of the DRBD 
disk. When I issue the pvcreate command, it works, but pvdisplay gives 
me the following:

# pvcreate -vv /dev/drbd1
       Setting global/locking_type to 1
       File-based locking selected.
       Setting global/locking_dir to /var/lock/lvm
       Locking /var/lock/lvm/P_orphans WB
       /dev/drbd1: No label detected
       /dev/drbd1: size is 2914640640 sectors
       metadata/pvmetadatasize not found in config: defaulting to 255
       metadata/pvmetadatacopies not found in config: defaulting to 1
       /dev/drbd1: size is 2914640640 sectors
     Set up physical volume for "/dev/drbd1" with 2914640640 available 
       Scanning for labels to wipe from /dev/drbd1
     Zeroing start of device /dev/drbd1
       Writing physical volume data to disk "/dev/drbd1"
       /dev/drbd1: Writing label to sector 1
   Physical volume "/dev/drbd1" successfully created
       Unlocking /var/lock/lvm/P_orphans

# pvdisplay /dev/drbd1
   "/dev/md1" is a new physical volume of "1,36 TB"
   --- NEW Physical volume ---
   PV Name               /dev/md1
   VG Name
   PV Size               1,36 TB
   Allocatable           NO
   PE Size (KByte)       0
   Total PE              0
   Free PE               0
   Allocated PE          0
   PV UUID               cRN1vb-T3pe-x85L-Ww0I-9DXf-7cSF-d6PAzW

Yes, you read that right. It says /dev/md1. At this point I'm not sure 
if this is a minor naming bug, or a major data corruption bug? I'm sure 
that if the device is changed without updating the metadata, it will 
potentially destroy data.

Can anybody explain this? I sure looks like a bug in the block layer, 
but is it related to DRBD? Or is is entirely benign?

Med venlig hilsen / Best regards
Christian Iversen

Sikkerhed.org ApS
Fuglebakkevej 88                       E-mail:  support at sikkerhed.org
1. sal                                 Web:     www.sikkerhed.org
DK-2000 Frederiksberg                  Direkte: ci at sikkerhed.org

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