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Re: [linux-lvm] Software raid on top of lvm logical volume



I've done something like Markus explained. I had this email ready to go a couple days ago, but the compose window was hidden for a while... I hope it's not too stale for the original poster of this thread:

2.4.26 with latest LVM2 works well. I did something crazy, alot of people probably don't understand why.

I have software RAID5.

md0 = hda5 hdb5 hdc5 hdd5 hde5
md1 = hda6 hdb6 hdc6 hdd6 hde6
md2 = hda7 hdb7 hdc7 hdd7
md3 = hda8 hdb8 hdc8 hdd8

Then I have just one big vgdata and one lvdata.

Why would I want to do this? I couldn't find any other way to have redundancy and be flexible. If I replace a physical disk with a larger one, for instance hde needs to be bigger so I can make md2 and md3 stretch across that disk too... then I am able to do pvmove from md2 to a scratch disk, recreate md2 across hda7-hde7, then pvmove back.

-Scott

Markus Baertschi wrote:

Michael T. Babcock wrote:


I've thought about this numerous times -- there is the distinct resizing advantage. Namely, if I create a software RAID partition, I can't resize it afterward without destroying it. I have for example, on occasion, had three disks set up where 1/3 of each was devoted to a RAID-0 very fast striping set for data transfers that had to be fast but if they were lost it wasn't critical, and 2/3 was set up as RAID-5 for reliability of another set of data.


I understand this may not be optimal in some situations, but some of us don't have thousands of dollars for SCSI HW RAID controllers.


You can make life easier by creating several raid arrays on the same disks and tie them together with LVM.
For example split your three 160GB drives into 4 partitions each and create a raid over the three disks.


sda -> sda1+sda2+sda3+sda4, etc.
md0 = sda1+sdb1+sdc1, md1=sda2+sdb2+sdc2, etc.

Markus



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