How to configure software RAID on Dell Express Flash PCIe SSDs

How to configure software RAID on Dell Express Flash PCIe SSDs

Applies to: 
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Operating System(s) - Oracle Linux 6.x, RHEL 6.x

Server Platform(s) - PowerEdge R720, R820

Author:
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Naveen Iyengar

Problem: 
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How to configure software RAID on Dell Express Flash PCIe SSDs

Solution: 
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1. Identify the Express Flash block devices– Dell’s Micron Express Flash drives show up as the following block devices in EL6.x OS

$> cat /proc/partitions

Major minor #blocks name
251    256     341873784  rssda
251    512     341873784  rssdb

2. Create a Partition – Use the fdisk linux utility as follows to create an 'fd' type partition on the Flash drives

$> fdisk –u /dev/rssda

Command (m for help): n
Command action
  e extended
  p primary partition (1-4)
p

Partition number (1-4): 1
First sector (56-683747567, default 56: 128
Last sector, +sectors or +size…, default 683747567: <hit Enter> 

Command (m for help): t
Selected partition 1
Hex code (type L to list codes): fd
Changed system type of partition 1 to fd (Linux raid autodetect) 

Command (m for help): wq
The partition table has been altered!
$>

3. Create software RAID– Use the Linux utility tool called mdadm as follows to create the software RAID array on the Express Flash drives. The following example create a RAID1 using two Flash drives /dev/rssda1 and /dev/rssdb1.

$> mdadm --create /dev/md0 --level=mirror --raid-devices=2 /dev/rssd[ab]1

4. Viewing the details of the array: View the status of the multi disk array md0.

$> mdadm --detail /dev/md0

5.  Make the array persistent across reboots: To add md0 to the configuration file so that it is recognized next time on boot, do the following:

$> mdadm -Es | grep md0

Check if the above command displays the details of the md0 array created. If not, try

$> mdadm -Es | grep “md/0”

Depending on which of the above two command works, run the appropriate command below to add info to the mdadm.conf file

$> mdadm -Es | grep md0 >> /etc/mdadm.conf

Or

$> mdadm -Es | grep “md/0” >> /etc/mdadm.conf

6.  Check for resync complete: Run the --detail option to make sure that the two SSDs in the array are not in the Resync Status, or wait until it finishes resyncing before you run a test against them.

$> mdadm --detail /dev/md0

A.    Deleting the array:

1. To halt the array:
   $> mdadm --stop /dev/md0
2. Delete the superblock from all drives in the array
   $> mdadm --zero-superblock /dev/sd[abc]1
3. Edit /etc/mdadm.conf to delete any rows related to the deleted array(s)

 Atthecommandprompt,type:

1.   #>fdisk/dev/<block_device>

   where <block_device> isthenameoftheblockdevicethatyouareadjusting.Forexample,iftheblockdeviceis /dev/sdb,type: fdisk/dev/sdb

   Thesystemdisplaysthefollowingmessage:

   Thenumberofcylindersforthisdiskissetto8782.Thereisnothingwrongwiththat,butthisislargerthan1024,andcouldincertainsetupscauseproblemswith:
   1)softwarethatrunsatboottime(e.g.,oldversionsofLILO)2)bootingandpartitioningsoftwarefromotherOSs 
   (e.g.,DOSFDISK,OS/2FDISK)
   
       a. Command(mforhelp):x #ToentertheexpertMode
           b.  Expertcommand(mforhelp):b #Tomovebeginningofdatainapartition
           c.  Partitionnumber(1-4):1 #Thepartitionnumbertobealigned
           d.  Newbeginningofdata(128-xxxxx,default128):128
   
   
   

   NOTE	128blocksor64KBisthedefaultStripeElementSizeofEqualLogicPSSeriesandPowerVaultMDLineofstoragearrays.Henceweadjustthebeginningofthedatapartitionto128blocks.

   
       e. Expertcommand(mforhelp):wq #writeandquit
   

2. Repeatstep1forallthedisksthatneedtobealigned 
   

3. Runthefollowingcommandtore-scanallthepartitions:

   #>partprobe
   

4. Verifythatthepartitionhasbeenalignedbyrunningthefollowingcommand:
   

   #>fdisk-ul/dev/<block_device>

   where <block_device> isthenameoftheblockdevicethatwealignedthepartitionof.Belowisanexampleofasampleoutputoftheabovecommandonablockdevicethathasbeenaligned.Ifyoupartitionisproperlyalignedthenyouwillsee 128 undertheStartcolumnagainstyourpartition.
   
   

   Disk/dev/mapper/mpath70:53.6GB,53697576960bytes
   255heads,63sectors/track,6528cylinders,total104878080sectors
   Units=sectorsof1*512=512bytes
   DeviceBoot      Start         End      Blocks   Id  System

   /dev/mapper/mpath70p1  128   104872319    52436096   83  Linux

5. Ifrequired,rebootthesystemforthekerneltobeabletore-readthenewpartitiontableorifyoudonotseeyournewlycreatedalignedpartition.