mirroring disk with unequal speed

[Kenneth_42]

Hi all,

Is it posible to mirror a faster speed disk with that of a lower one?

The setup is: HP4440 with one internal 73G disk (VG00) running at 15K RPM. It has two external disk (VG01) which are mirrored on an external disk array, both 73GB disk running at 10K RPM. The two disk currently is not being used.

What I want is to use the two external disk and use it as the inetrnal disk of the server in a mirrored config. I intend to use the 15K internal disk as a mirror disk for another exactly configured server.

If it's possible? can someone please provide some steps on how I can take away the external disk out of the filesystem.

Thanks in advance!

[Rajeev Shukla · Accepted solution]

I am sorry but your questions dont seem to be too specific to me. Ok let me understand and let me know if i am wrong.
1. You asked if disks with different speed can be mirrored. Yes they can be mirrored the only problme is you will have some performance issue, but not significant.
2. You want to mirror your external disk with the internal disk and then use that mirrored disk as a boot disk of another system (tell me if i didnt get you). Dont do that you will run into lots of problems, system complaining about lvmtab out of sync etc..
3. To take away your external disk out of the system, i.e right now it belongs to vg01.
Do a vgdisplay -v vg01 and see what LV are created on that disk. unmount all LV (if mounted as filesystem) and do a lvremove to remove the Logical volumes.
The do a vgremove (if only one disk in vg01) or else do a vgreduce and remove the external disk out of vg01.

[Bill Hassell]

Yes, you can have differing speeds for mirrors. Even disks that are the same speed are not exactly the same speed so the mirroring driver waits for the last disk to complete it's I/O. There are some sanity checks to make sure that all I/O's are complete before moving on. These sanity checks will assume a disk is dead if the time is excessive, so you can't mirror a fast disk to a floppy with any success.

That said, there is a performance concern. While reads will proceed at full speed (no need to read the mirror if the primary is working), writes will always complete at the slowest disk's rate. So overall system performance will be limitged somewhat by the slowest disk. But it will mirror OK.

[Devender Khatana]

Hi,

As mentioned in above posts it can be achived but by sacrifying the writes at the speed of the low RPM disk.

But what you are trying to achive is not requiring to have two disks mirrored at different speed. If I have understood you requirement correctly you want to remove two external 10K RPM disks and then use them as internal mirrored disks in one system & two internal 15K RPM disks from two systems as mirrored disks in another system. The only requirement in this case will be to mirror only at one instance as you want to replace the existing 15K RPM disk from any one system with 10K RPM disk. So only for this much I do not think it will effect anything.

Procedure for removing the configured disks in vg01 -
1. #bdf ( To confirm no vg01 volume is mounted)
2. If mounted unmount it and remove entry from /etc/fstab for mounting the file system at reboots.
3. #vgexport /dev/vg01

4. If the raid is through a raid controller then delete the logical drive created through the controller. ( Procedure should be as per the raid controller manual / model).

5. Confirm removal of VG from system by "strings /etc/lvmtab".

Now these disks can be used anywhere internally or externally.

HTH,
Devender

[Kenneth_42]

Thanks, got the information I wanted! Another question to be raised in another thread!

[Kenneth_42]

Just to clear things up. I have two identical servers server1 and server2 each with one 15K RPM internal disk and two 10K external disks (the two external disk are mirrored).

Since server2 does not use its two external disk and performance wise is not as critical as server1, I plan to take out the internal disk in server2 to be used as a mirror disk for server1.

So I plan first to mirror one of the slower disk in server2. After it is mirrored, I'll pull out the 15K RPM internal disk and put in place the other 10K RPM disk.

Anyway, thanks guys as I got the answers I needed from you.

Regards,
Kenneth