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Old 05-12-2010, 12:18 PM   #1
IwantLINUX
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Registered: Oct 2004
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SCSI/Fiber Devices Naming


Hi,

Need help on how the scsi and multipathing works in Linux. From the docs i have read, i understand that by the use of multipathing we can assign multiple paths to a SAN partition. If there is a problem then one path will failover to other path.

However, i am not clear on how linux recognizes the SAN partitions using the multipath drivers. For e.g. I have a HP Proliant server on which we have the following mounts:

Filesystem Size Used Avail Use% Mounted on
/dev/cciss/c0d0p3 59G 11G 46G 20% /
/dev/cciss/c0d0p1 494M 27M 443M 6% /boot
none 3.9G 0 3.9G 0% /dev/shm
/dev/cciss/c0d1p1 673G 307G 333G 48% /oraback
/dev/mapper/oravg-oradata
296G 116G 165G 42% /oradata

/dev/cciss/c0dx are two SAS RAID drives which are local and /dev/mapper/oravg-oradata is SAN partition on which LVM has been created. Now when I look under /proc/scsi/scsi I have the following:

Host: scsi0 Channel: 00 Id: 00 Lun: 00
Vendor: IBM Model: 2810XIV-LUN-0 Rev: 10.0
Type: RAID ANSI SCSI revision: 05
Host: scsi0 Channel: 00 Id: 00 Lun: 01
Vendor: IBM Model: 2810XIV Rev: 10.0
Type: Direct-Access ANSI SCSI revision: 05

Why only two SCSI devices are visible. The local drives are SAS drives, shouldn't they should also be visible under /proc/scsi/scsi?

For the SAN drive, when I give: multipath -ll it shows the following output:

mpath0 (20017380000f10067)
[size=304 GB][features="0"][hwhandler="0"]
\_ round-robin 0 [active]
\_ 0:0:0:1 sda 8:0 [active]

AFAIK, it means that SAN drive has been detected as mpath0 and is assigned block device /dev/sda. When i see fdisk output it shows the following:

Disk /dev/sda: 326.4 GB, 326417514496 bytes
Disk /dev/sda doesn't contain a valid partition table

Disk /dev/dm-0: 326.4 GB, 326417514496 bytes
Disk /dev/dm-0 doesn't contain a valid partition table

Disk /dev/dm-1: 322.1 GB, 322122547200 bytes
Disk /dev/dm-1 doesn't contain a valid partition table

I am confused, how /dev/dm-1 got created as there is only /dev/mpath/mpath0 -> ../dm-0

"pvs" command show the following:

[root@arisjjprddb01 ~]# pvs
PV VG Fmt Attr PSize PFree
/dev/dm-0 oravg lvm2 a- 303.88G 3.88G

Please help.

Regards,
 
Old 05-12-2010, 12:30 PM   #2
TB0ne
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Not sure what to help WITH. Looks like your SAN is working fine. You don't say anything about your version/distro of Linux, your HBA's, or what you're wanting, or what problem you're having.

What's visible from the HBA's is whatever your SAN guys presented you. How you've got things configured will also be a factor. We can muddle through it here, but you're in a commercially-supported environment. Your SAN team, IBM, software vendor, and your HBA manufacturer, all have support for whatever you're trying to do. IBM would be my first call in your case, since they know how your environment is set up. They can probably answer in 10 minutes, what would take us days.
 
Old 05-12-2010, 12:49 PM   #3
IwantLINUX
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Thanks TB0ne!!

Well you are very much right its a professional environment and with all support. However, we are not allowed to register a case directly with IBM without taking our senior techs loop and you know senior techs are always busy for our questions..that's why i posted it here as i have seen people extending their full support on this forum. There no implementation issue and things are working fine, i just want to enhance my knowledge on how linux assign names to fiber devices when multipathing is used.

Actually, till now i have worked with local SCSI drives and linux detects them fine under /proc/scsi/scsi and you get /dev/sda /dev/sdb etc ...however this is first time when i looking at SAN.

Linux version in RHEL 5.4 and HBA card is from Qlogic.
 
Old 05-12-2010, 02:17 PM   #4
TB0ne
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Quote:
Originally Posted by IwantLINUX View Post
Thanks TB0ne!!

Well you are very much right its a professional environment and with all support. However, we are not allowed to register a case directly with IBM without taking our senior techs loop and you know senior techs are always busy for our questions..that's why i posted it here as i have seen people extending their full support on this forum. There no implementation issue and things are working fine, i just want to enhance my knowledge on how linux assign names to fiber devices when multipathing is used.

Actually, till now i have worked with local SCSI drives and linux detects them fine under /proc/scsi/scsi and you get /dev/sda /dev/sdb etc ...however this is first time when i looking at SAN.

Linux version in RHEL 5.4 and HBA card is from Qlogic.
Yeah, I know how it can be in some big environments. But still, it's the job of the sr. techs to pass their knowledge along. If they're not, they're not doing their jobs.

To your question: it depends. Usually SAN devices have a 'gatekeeper' device that'll show up, no matter how many LUN's are presented from the SAN. The gatekeeper is usually small, maybe only 1 or 2 GB. If you're presented two LUN's, how they'll show up depends on how the configuration goes. They may show up as two devices, or if you've got them in an LVM or Multipath, may show up as ONE device, with TWO paths to it. All this depends on your software. Veritas is different from Sun's solution, different from IBM, and Linux too. There's really no hard-and-fast answer.
 
  


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