topic Re: SAN switch host reassignment in MSA Storage

SAN switch host reassignment

SWAP — Fri, 20 Feb 2009 06:44:40 GMT

I am new member to this group so applogise for silly query if any.

I have MSA2000, only one controler(4.2 MSA2012FC) & one FC switch (SAN Switch 4/8). I have created 3 LUNs & they are maped to two hosts.

current case:
Host LUN FC Port 0 FC Port 1
HostA 0 Read-write Read-write
HostA 1 Read-write Read-write
HostA 2 Read-write Read-write
HostB 0 Read-write Read-write
HostB 1 Read-write Read-write
HostB 2 Read-write Read-write

Plan to do is

Host LUN FC Port 0 FC Port 1
Host A 0 Read-write no access
Host A 1 no access no access
Host A 2 Read-write no access
Host A 0 no access no access
Host A 1 no access Read-write
Host A 2 no access Read-only

Please reply is this possible? & how do I go abt it?

awaiting reply.

Re: SAN switch host reassignment

Uwe Zessin — Fri, 20 Feb 2009 08:10:28 GMT

I guess the last 3 entries in your plan mean 'Host B' and it looks like you want to allow a fast means to provide data from Host A to Host B.

You don't write which operating system you use, but this almost never works:
- some file systems cannot be mounted read-only
- changes by Host A will not be seen by Host B 'immediately', because B assumes exclusive access to the volume, too. It does not expect that another server makes changes

Do NOT allow read-write access for both hosts unless you *know* you have a file system that really allows sharing of volumes. Please do *NOT* experiment! You DO risk losing data!

Re: SAN switch host reassignment

SWAP — Mon, 23 Feb 2009 09:57:18 GMT

Hi Uwe Zessin,

Thanks for your valuable inputs.

I am detailing you with exact problem we are facing.

This is how our SAN was configured previously.

Host OS: WIN2K3 R2 Ent sp2

Host LUN FC Port 0 FC Port 1

Host A 0 Read-write Read-write
Host A 1 Read-write Read-write
Host A 2 Read-write Read-write

Host B 0 Read-write Read-write
Host B 1 Read-write Read-write
Host B 2 Read-write Read-write

Above configuration caused data corruptions on Lunâ ss as rightly said by you.

Current situation:- Host B access is dropped by disabling switch port & corresponding SAN port.

As a part of our change design which needs to be implemented, we have decided to reconfigure Host access as below.

Host LUN FC Port 0 FC Port 1

Host A 0 Read-write no access
Host A 1 no access no access
Host A 2 Read-write no access

Host B 0 no access no access
Host B 1 no access Read-write
Host B 2 no access Read-only

My worries are as below.

1) while doing this(reassignment) do I risk data loss? (Lun 1 which is currently used by host A will be used by host B after changes)
2) As LUN2 have read-write access for host A & read-only for Host B, will there be a time lag between data creation & data viewing especially from read only host.
3) Currently a switch FC port 5 is disabled for Host B, do I need to reactivate it before doing all these above changes?

Re: SAN switch host reassignment

Pieter 't Hart — Mon, 23 Feb 2009 10:58:02 GMT

Hi there,
Like Uwe allready said.
sharing SAN-disks among servers that know nothing about eachother is a BAD idea.
This only works when some form of clustering software is used.

with windows-clustering, both hosts KNOW the disk is shared, so when one host is controlling the disk, the other keeps off!.

with an OS like tru64cluster there is a lock-manager, so the disk can really be shared among cluster-nodes.

depending on the windows-driver used and with two ports enabled you have either a failover-connection or a load-sharing connection
using only a single FC-port gives you no advantage.

Re: SAN switch host reassignment

SWAP — Tue, 24 Feb 2009 05:10:43 GMT

HI Pieter,

Thanks for your guidance.

I understand that you are talking about continuity point of view as one switch will not advantage & I will speak with seniors to go for another switch but we can't go hardware/software procurement in current scenario.

The current configuration is already messed-up & we have come out with the below solution with available hardware

Host LUN FC Port 0 FC Port 1

Host A 0 Read-write no access
Host A 1 no access no access
Host A 2 Read-write no access

Host B 0 no access no access
Host B 1 no access Read-write
Host B 2 no access Read-only

Please help me understand whether this solution will work or I am in a risk of loosing data.

Re: SAN switch host reassignment

SWAP — Tue, 24 Feb 2009 12:00:00 GMT

Hi All,

Please help me to resolve this isssue.

Re: SAN switch host reassignment

Uwe Zessin — Tue, 24 Feb 2009 12:17:20 GMT

1) changing a volume's presentation always is a risk. Best is if you make a full backup and then shut down the server. This makes sure that the file system is closed safely and there is no risk of data loss due to a disruption in connectivity.

2) There is no 'time lag'. Host B simply does not 'know' that the data can be updated by a different host so it will not attempt to refresh its file system view. It is even worse - due to unsynchronized access, host B could see inconsistencies which _can_ result in a crash of host B's operating system!

If you want host B to be able to access host A's data - use a simple file share.

3) No, volume presentation is completely independent from that.

Re: SAN switch host reassignment

SWAP — Thu, 26 Feb 2009 09:19:14 GMT

Thanks a lot Uwe Zessin,

I have finally got the solution.

Re: SAN switch host reassignment

SWAP — Mon, 23 Mar 2009 08:06:05 GMT

Hi Uwe Zessin,

Host LUN FC Port 0 FC Port 1

HostA 0 Read-write no access
HostA 1 no access no access
HostA 2 Read-write no access
HostA 0 no access no access
HostA 1 no access Read-write
HostA 2 no access Read-only

After performing above change I observed that LUN 2 has Read-write access from HostA but when I mapped same LUN to hostB as read-only as mentioned in above chart 3rd & 6th line, it' not accessible from hostB.

It shows as drive in my computerbut do not display properties & while trying to access gives error as"No media is write protected"

Re: SAN switch host reassignment

SWAP — Mon, 23 Mar 2009 08:21:01 GMT

Sorry the error is "The media is write protected"

Re: SAN switch host reassignment

Uwe Zessin — Mon, 23 Mar 2009 08:56:35 GMT

As far as I can tell, NTFS can only be mounted when the volume allows read/write access. Watch out! I have seen Windows 2000 systems hanging during boot when one of their data disks was set readonly!

Again, you cannot have two servers access one NTFS volume properly (even if the second one could have a read-only view) - please re-read my first response.

Re: SAN switch host reassignment

SWAP — Tue, 24 Mar 2009 03:46:17 GMT

HI Uwe Zessin,

Thanks a lot for helping me in understanding this concept. As this change is already done & this is project requirment, what else is the option?

Can Switch zoaning solve this issue?

We have 16 port Brocade Fibre Channel Switch.

Please reply if addtional information is needed.

Re: SAN switch host reassignment

Uwe Zessin — Tue, 24 Mar 2009 04:57:11 GMT

No, sorry, switch zoning cannot help, because it is used to block complete FC-ports from communicating with each other.

You could look if it is OK if hostB goes to hostA's data via a simple file share. Otherwise you need a shared file system add-on for Windows, like HP's PolyServe software.

Re: SAN switch host reassignment

SWAP — Fri, 24 Apr 2009 06:15:08 GMT

Hi Uwe,

It was great help from you in resolving this problem.

Can you though more light on how exactly Changing a volume's presentation to be carried out.

Thanks in advance.