MCSG with two like servers, two FC60s and raw devices !!!

[Henrique Silva_3]

This is a question that came up on our lsat install.

Our sybase database has its data files on raw devices, so, AFAIK, HP does not have anything that would give us raw device mirroring on the FC60s.

So, we decided to use only one FC60, connect both servers to it, but now, that is our single point of failure.

We entertain the idea of having sybase replication going, but this started to become TOO COMPLEX and our managers dropped it.

Now, has anybody out there encountered this scenario, and how did you get around it ?

Thanks,

Henrique Silva

[A. Clay Stephenson]

Hi:

There is a way to do what you want to do and that is to use raw LVM lvols. These, you can mirror. I have never been able to measure any significant difference in the performance of raw physical devices and raw logical volumes; in fact, because you can stripe across multiple physical devices, in some cases, the LVM raw LVOL's actually perform better.

[harry d brown jr]

Why not let sybase use a raw Logical Volume, which you can mirror?

live free or die
harry

[Henrique Silva_3]

OK, I am not familiar with the RAW LVM !!!!!

Can you clarify this ? At the OS level or sybase level ??? I have already create raw LVMs for sybase raw devices. That is what I meant of raw devices. Now, I am not aware of any way to mirror them at the OS level.

How is that done and what package does it ? This would have to be hardware or HP Software mirroring or you are telling me that sybase would take care of that ? I am not aware that sybase can mirror its raw devices on two servers and two fc60s. I know it can mirror the devices on the same server, but not accross servers, without doing replication.

please clarify it. I am REALLY curious here now :-)))

Thanks,

Henrique

[A. Clay Stephenson · Accepted solution]

Search for a product called Mirror/UX. The mirroring is done at the OS level and works quite well. If you are on a later OS Release, yiu should also look at a product called Veritas Volume Manager (VxVM). Both of these are add-on products and will do the trick for you.

I might also suggest another trick that I have found to be useful. Add a layer of indirection to your database raw volumes. e.g. Rather that using /dev/rdsk/c9t6d3 (or /dev/vg03/rlvol5) as your raw volume, use something like /u01/myfile.dat and then do a symbolic link to the raw device node. This wa, you can move things around at the UNIX level and not have to change anything in the database. Simply redefine your soft link and the data is moved.

Food for thought, Clay

[A. Clay Stephenson]

Ooops ... that should be MirrorDisk/UX.

[manoj_pu]

Hello Silva

You can do mirroring by using mirror/UX aswellas by sybase isql command.

LVM mirroring
Assumptions
vgXX --- Volume group with two LUNS.
eg. cXtXd0 & cXtXd1
lvolX -- Logical volume where your sybase device resides its primary copy is in cXtXd0
To create a mirror copy of lvolX in cXtXd1
#lvextend -m 1 /dev/vgXX/lvolX /dev/dsk/cXtXd1

Sybase mirroring

You will have to create a device of same size of primary database device.

Then use disk mirror command of sybase to enable mirroring in the devices.

This will ensure that the data is written to both devices at the same time


Regards

Manoj

[A. Clay Stephenson]

Hi again:

I suppose that should add that you use the -m n options of the lvextend command but this option only is available if you have purchased and installed MirrorDisk/UX. Man lvextend for details.

[Sanjay_6]

Hi Henrique,

since you are using a Disk Array, so we can safely assume that you are using some RAID Level, so you already have a hardware redundancy built in. Software mirroring add some more redundancy.

Take a look at the threads below to learn more about mirroring and redundancy,

http://docs.hp.com/cgi-bin/fsearch/framedisplay?top=/hpux/onlinedocs/B2355-90742/B2355-90742_top.html&con=/hpux/onlinedocs/B2355-90742/00/00/98-con.html&toc=/hpux/onlinedocs/B2355-90742/00/00/98-toc.html&searchterms=lvm%20mirror&queryid=20020223-200258

http://docs.hp.com/cgi-bin/onlinedocs.py?mpn=B2355-90742&service=hpux&path=../B2355-90742/00/00/96&title=Managing%20Systems%20and%20Workgroups%3A%20A%20Guide%20for%20HP-UX%20System%20Administrators

You can use software mirroring is you have MirrorDisk/UX installed on your system.

Hope this helps.

Regds

[Henrique Silva_3]

Let me just make sure we are ALL talking about the same thing here.

The RAW device mirroring would be done BETWEEN TWO SERVERS and not on the same server. This would be done between server a and server b, bith have FC60s attached to them an d mirrored. Tha SG Package is called SERVERS.

When server A is active, everything is written to the disk array ( FC 60 ) on server B. The same thing happens the other way around.

I am getting confused a bit here, because I think, people are talking about mirroring disks ON THE SAME SERVER. I know I can do this at the OS level or using sybase, althought I would pay with performance there. The
trick here is to have the raw device mirrored on the two SG nodes.

Are you saying I can do that with mirror / UX ? How does that work ? Will both FC 60s be mounted on both servers and them mirror/UX will think they reside on that server and mirroring is done it that way, or is this LAN mirroring ??????

Thanks,

Henrique Silva

[Jeff Schussele]

Henrique,

No, Mirror UX mirrors data (LVM or raw) across multiple (up to 4 I believe) disks on the SAME server - NOT across servers. These disks can be either internal to the server or external in an array. Furthermore the mirror can consist of drives from either. Although this definitely adds a layer of complexity & makes admin tougher obviously.

The earlier assertion of RAID redundancy is also correct. It is therefore possible to have only two "copies" of data actually residing across four separate "disks" if BOTH Mirror UX and a RAID are present. The SW (Mirror UX) handles one set of copies - while the HW (array) handles the other. Both are essentially "invisible" to the other.

MC/SG will allow ONLY one server to run any one pkg at any one time. The pkg "owns" the Volume Groups & therefore the server running the pkg "owns" the VGs. This prevents mirroring ACROSS servers.

Note, however, that you can have different pkgs running across different servers in the cluster at the same time. Just not the SAME pkg on multiple servers at the same time.

It also should be noted that you are entirely correct - and should be concerned - about your a single point of failure. Having a single FC60 in an MC/SG environment is inherently risky - almost to the point of defeating the entire purpose of MC/SG. Not only should you have 2 FC60s you should have dual-paths to both FC60s! FC cards fail too.

Hope I have not added to your confusion.

Jeff

[Henrique Silva_3]

Thanks Jeff, NO, you have clarified my confusion.

My original post was very poorly written,and looking over, I understand why people responded the way they did.

The original post should have said, SERVER A has an FC60 and server B has an FC60. We set up service guard between these two, but because the databases are on raw devices, we could not find a RELIABLE WAY to mirror the database.

We thought about sybase replication, but it very fast became too complex.

So, we decided to use only ONE FC60 connected to both servers. I do not want to duel too much ito it, but I brough up the single point of failure to my manager at the time, but she made the call to go with this setup, for the sake of time ( we were a bit behind on the project ).

Now, during testing, we DID use both FC60s, mounted to both servers, so that we could switch the server identity to work on each ( one of the servers was already on production and we were setting up the service guard as such ). And we would mount volume groups 0-10 from the first FC60 and 11-20 from the second one on both servers, so that we could exchange info on the fly.

I was wondering if we had such setup, if we could use Mirror/UX or veritas, to mirror the raw devices by mounting both FC60s to BOTH servers, and having the package access the proper FC60 in case of failure.

Hummnnnn, could we have done such thing and be succesfull ? MIrror UX would be working ONLY on the active server, and because the server had both FC60s mounted on it, the active server would have mirroring working. Maybe this is waht people refer to earlier !!!

Henrique

[Sandip Ghosh]

Henrique,

As you have purchased fc60 you dont have to do any Software mirroring . You can configure the FC60 Array as you like. It can be configured as RAID1 (Simple Mirror) or you can configure it as RAID5 , as you like. You only need to have a Fiber Optic Switch or HUB. Both the nodes and the FC60 will be connected over there. So from anywhere you can access anything.

You can get the detail about fc60 in the following pages
http://www.hp.com/cposupport/manindex/hpsurestor19163_eng_man.html

Sandip

[A. Clay Stephenson]

Hi:

Now that the problem is stated a bit more clearly then yes you can do the mirroring. The key is that both FC60's are connected to both hosts. You create a set of LUN's on both FC60's and these are made members of the same volume group. Each LVOL is mirrored on the other FC60. This could be done using LVM with Mirror/UX or with VxVM. The key is that the host running the package actually does the mirroring and has the LUN's on both FC60's mounted. Other LUN's could be created and used by the standby node or be used to run other packages. As mentioned, you should also set up primary/alternate paths from each host to each FC60. You could then tolerate the failure of an array or a host and your data would be fully replicated.

By the way, this same approach could also be used with copper SCSI.

Food for thought, Clay