hello
i'm using RHEL5.0 with kernel-2.6.18-8.el5 now i implement raid{0,1,5} on redhat installed pc but only one md device is available /dev/md0
so how i extend my md device for implement more raid device at one time..
note:-- i'm taking about SOFTWARE RAID.....

Without more context this question is weird to interpret.

md0 indicates a raid device already. It will be constructed based on raid physical devices.. if you want to implement raid{0,1,5}, saying all three of them, you will end up with an md0 md1 and md2, each of one of the three types. You don't need to extend an md0 device per say to achieve this.

If you are talking about an md0 device, covered on multiple physical disks that are bigger than the area designated to the md0 device, you should resize all partitions on all physical drives and try to use mdadm --grow, if at all possible..

If this does not answer your question, please rephrase or explain some more your situation.

actully sir
in RHEL5 when i implement software any raid such as raid5
1:--create 3 partition /dev/hdc{8,9,10} with fd id then run partprobe for kernel reinitialize.
2:--mdadm --create /dev/md0 --level=5 --raid-devices=3 /dev/hdc{8,9,10}
now array created on /dev/md0 device
3:--mkfs.ext3 /dev/md0
now create a directory then set the mount point ,write entry in /etc/fstab
for parmanent mounting.
now i want at this runing linux pc one more raid 1 implement
1:--i create two more partition such as /dev/hdc{11,12} with fd id then run partprobe for kernel reinitialize.
2:--mdadm --create /dev/md1 --level=1 --raid-devices=2 /dev/hdc{11,12}
now array not creat on /dev/md1 device
one error message is showing "/dev/md1" device is not availabel
so now what i do..
this means how to extend md device ....
but when i used RHEL4 then 3raid device such as raid 0,1,5 was implemented
at a time..on only one runing rhel pc

Just got to ask. What is the point of creating a raid array on a single disk? You do not increase speed. If the drive has a failure, you do not have any backup.

I suppose this is for some testing purpose, on which it is totally understandable. If not, like lazlow says, it is rather useless, neither performance, nor safety will be increased..

I would have to look into this myself. I will do so though...

Technically (that made it unclear to me) you are not exactly extending an existing md device, but rather you want to create an additional md device...