Prior to coming to the Linux world, most of my complex, software-based storage taskings were performed under the Veritas Storage Foundation framework. In recent years, working primarily in virtualized environments, most storage tasks are done "behind the scenes" - either at the storage array level or within the context of VMware. Up until today, I had no cause to worry about converting filesystems from using one underlying RAID-type to another.
Today, someone wanted to know, "how do I convert from a three-disk RAID-0 set to a six-disk RAID-10 set". Under Storage Foundation, this is just an online relayout operation - converting from a simple volume to a layered volume. Until I dug into it, I wasn't aware that LVM was capable of layered volumes, letalone online-conversion from one volume-type to another.
At first, I thought I was going to have to tell the person (since Storage Foundation wasn't an option for them), "create your RAID-0 sets with `mdadm` and then layer RAID-1 on top of those MD-sets with LVM". Turns out, you can do it in LVM (I spun up a VM in our lab and worked through it).
Basically the procedure assumes that you'd previously:
When you're ready to do the conversion from RAID, add second set of identically-sized disks to the system. Format the new devices and use `vgextend` to add the new disks to the volumegroup.
Note: Realistically, so long as you increase the number of available blocks in the volumegroup by at least 100%, it likely doesn't matter whether you add the same number/composition of disks to the volumegroup. Differences in mirror compositions will mostly be a performance rather than an allowed-configuration issue.
Once the volumegroup has been sufficiently-grown, use the command `lvconvert -m 1 /dev/<VolGroupName>/<VolName>` to change the RAID-0 set to a RAID-10 set. The `lvconvert` works with the filesystem mounted and in operation - technically, there's no requirement to take an outage window to do the operation. As the `lvconvert` runs, it will generate progress information similar to the following:
Once the conversion has completed, you can verify that your RAID-0 set is now a RAID-10 set with the `lvdisplay` tool:
Today, someone wanted to know, "how do I convert from a three-disk RAID-0 set to a six-disk RAID-10 set". Under Storage Foundation, this is just an online relayout operation - converting from a simple volume to a layered volume. Until I dug into it, I wasn't aware that LVM was capable of layered volumes, letalone online-conversion from one volume-type to another.
At first, I thought I was going to have to tell the person (since Storage Foundation wasn't an option for them), "create your RAID-0 sets with `mdadm` and then layer RAID-1 on top of those MD-sets with LVM". Turns out, you can do it in LVM (I spun up a VM in our lab and worked through it).
Basically the procedure assumes that you'd previously:
- Attached your first set of disks/LUNs to your host
- Used the usual LVM tools to create your volumegroup and LVM objects (in my testing scenario, I set up a three-disk RAID-0 with a 64KB stripe-width)
- Created and mounted your filesystem.
- Gone about your business.
Take special note that there are free PEs available in the volumegroup. In order for the eventual relayout to work, you have to leave space in the volume group for LVM to do its reorganizing magic. I've found that a 10% set-aside has been safe in testing scenarios - possibly even overly generous. In a large, production configuration, that set-aside may not be enough.# vgdisplay AppVG --- Volume group --- VG Name AppVG System ID Format lvm2 Metadata Areas 3 Metadata Sequence No 2 VG Access read/write VG Status resizable MAX LV 0 Cur LV 1 Open LV 0 Max PV 0 Cur PV 3 Act PV 3 VG Size 444.00 MB PE Size 4.00 MB Total PE 111 Alloc PE / Size 102 / 408.00 MB Free PE / Size 9 / 36.00 MB VG UUID raOK8i-b0r5-zlcG-TEqE-uCcl-VM3L-RelQgX # lvdisplay /dev/AppVG/AppVol --- Logical volume --- LV Name /dev/AppVG/AppVol VG Name AppVG LV UUID 6QuQSv-rklG-pPv6-Tq6I-TuI0-N50T-UdQ4lu LV Write Access read/write LV Status available # open 1 LV Size 408.00 MB Current LE 102 Segments 1 Allocation inherit Read ahead sectors auto - currently set to 768 Block device 253:7
When you're ready to do the conversion from RAID, add second set of identically-sized disks to the system. Format the new devices and use `vgextend` to add the new disks to the volumegroup.
Note: Realistically, so long as you increase the number of available blocks in the volumegroup by at least 100%, it likely doesn't matter whether you add the same number/composition of disks to the volumegroup. Differences in mirror compositions will mostly be a performance rather than an allowed-configuration issue.
Once the volumegroup has been sufficiently-grown, use the command `lvconvert -m 1 /dev/<VolGroupName>/<VolName>` to change the RAID-0 set to a RAID-10 set. The `lvconvert` works with the filesystem mounted and in operation - technically, there's no requirement to take an outage window to do the operation. As the `lvconvert` runs, it will generate progress information similar to the following:
Larger volumes will take a longer period of time to convert (activity on the volume will also increase the time required for conversion). Output is generated at regular intervals. The longer the operation takes, the more lines of status output that will be generated.AppVG/AppVol: Converted: 0.0% AppVG/AppVol: Converted: 55.9% AppVG/AppVol: Converted: 100.0%
Once the conversion has completed, you can verify that your RAID-0 set is now a RAID-10 set with the `lvdisplay` tool:
The addition of the "Mirrored Volumes" line indicates that the logical volume is now a mirrored RAID-set.lvdisplay /dev/AppVG/AppVol --- Logical volume --- LV Name /dev/AppVG/AppVol VG Name AppVG LV UUID 6QuQSv-rklG-pPv6-Tq6I-TuI0-N50T-UdQ4lu LV Write Access read/write LV Status available # open 1 LV Size 408.00 MB Current LE 102 Mirrored volumes 2 Segments 1 Allocation inherit Read ahead sectors auto - currently set to 768 Block device 253:7
