MSA Storage

SAN switch host reassignment

 
SOLVED
Go to solution
SWAP
Advisor

SAN switch host reassignment

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.







13 REPLIES 13
Uwe Zessin
Honored Contributor
Solution

Re: SAN switch host reassignment

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!
.
SWAP
Advisor

Re: SAN switch host reassignment

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?


Pieter 't Hart
Honored Contributor

Re: SAN switch host reassignment

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.
SWAP
Advisor

Re: SAN switch host reassignment

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.
SWAP
Advisor

Re: SAN switch host reassignment

Hi All,

Please help me to resolve this isssue.
Uwe Zessin
Honored Contributor

Re: SAN switch host reassignment

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.
.
SWAP
Advisor

Re: SAN switch host reassignment

Thanks a lot Uwe Zessin,

I have finally got the solution.
SWAP
Advisor

Re: SAN switch host reassignment

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"
SWAP
Advisor

Re: SAN switch host reassignment

Sorry the error is "The media is write protected"