Multiple mirror copy

[Umang Bhavsar]

I am trying to find what would be the advantage of multiple mirror copies. I have 4 internal HD in rp8400. During backup we would split mirror and back it up. But during this time if one harddrive crash would the system would still be up or crash. We use just 2 drive as a mirror but bydefault when we got a new system we had a /dev/vg00/crash drive have 2 mirror copies. one on primary and 2 mirror on secondary drive. i am wondering would it be an advantage of the three drive mirror instead of two ?

Thanks in advance

[Michael Tully]

Unless you have *data* on these internal disks that needs to be split off, why split it at all? I don't see much value in having a second mirror unless you wish to keep a copy in a external disk array. Most people who have a system of this size, have an external disk array for data. I have a few of these (split into two partitions) and I only have the OS on the internal disks which are mirrored. In answer to your question, the system would be of little use if your mirror is split off for backup and the primary disk dies in it's current state. It is unlikely it would happen, but there is always a possibility with a single point of failure.

[Tim Adamson_1]

If you are talking about 2-way mirror of vg00 then it comes down to how quickly you can replace a defective disk.

When 'normal' mirroring is used on vg00, if a disk dies the system will continue to run. You are meant to replace the dead disk at your earliest convenience. However, if the other disk dies, then the system will obviously crash.

If you implement 2-way mirroring, you avoid that scenario. You need to way up the business impact of a server outage against the likelihood of 2 disks dying. And while you are at it, I'll have this weekend's tattslotto numbers :-)

If you are talking about application data and 2-way mirroring, the same scenario applies. It all comes down to availability and is a business decision. Also, consider that you could easily shutdown the application, break one of the mirrors, restart the application and then happily backup your application data on the split mirror whilst still enjoying the benefit of having the live application data mirrored. When the backup finishes, you simply merge the split disks back into a 2-way mirror.


Tim.

[Jeff Schussele]

Hi Umang,

The advantage of a 2 mirror setup (3 copies total) is exactly what you think - safety.
They are best used in highly dynamic datafile situations like databases, e-mail datastores, etc. Anything that constantly changes to the point where there is no quiescence, YET requires constant uptime, can benefit from a 3 copy mirror. Simply because one can split off the third copy & back it up - static - w/o causing an SPOF in the process. You still have a mirror copy. When complete, resync it for the next cycle.
But the catch & there's *always* a catch, is that you should do this with *three* separate HBA pairs to those 3 copies if you're that adamant about avoiding SPOFs. Comes down to how much is the data worth.....

My 2 cents,
Jeff

[Umang Bhavsar]

Michael and Tim : No i am using internal drives for just O.S. I have /stand,/,/var,/usr/tmp/swap etc on internal drives. My data is on XP 1024 disk array. but my production system are very important and can not wait to restore from ignite DR tape. My application is Oracle Which run as VG11 to VG30.
Jeff : yes i am thinking about the safety only because my backups done using lvsplit and veritas netagent i was thinking about the scenario when backups done in night we don`t have system admin available to restore if that primary drive crash. Although i need to come from home and do restore from ignite but which will take while.

Thanks you very much for your replys.

[Sunil Sharma_1]

Hi,

If you use lvsplit to backup root disk then it's always batter to have 2 copy of mirror but in my understanding it's not required to take backup like tjis for volume group vg00 if you don't have any application or data apart of system related is installed there.
you can take ignite backup which will backup full vg00.
and 2 way mirroring is really not required.

Sunil

[Dave Wherry]

One other possible downside. The system will have to wait while all 3 writes complete - primary, mirror 1 and mirror 2. If any file system in vg00 is very active, it could cause a slow down in I/O performance.
On the other hand, if you keep vg00 clean with just the operating system, it should not be very busy with writes and could actually improve read performance. Reads will be services from the first available of your 3 images.
I'm assuming you are using make_tape_recovery. Have you looked at using make_net_recovery? It is faster than the tape.

[Dave Wherry]

One other possible downside. The system will have to wait while all 3 writes complete - primary, mirror 1 and mirror 2. If any file system in vg00 is very active, it could cause a slow down in I/O performance.
On the other hand, if you keep vg00 clean with just the operating system, it should not be very busy with writes and could actually improve read performance. Reads will be serviced from the first available of your 3 images.
I'm assuming you are using make_tape_recovery. Have you looked at using make_net_recovery? It is faster than the tape.