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what happens if non-root LV is not mirrored

 
masood bt
Regular Advisor

what happens if non-root LV is not mirrored

HPUX 11.11

We have VG00. two PVs - mirrored.

Created a filesystem in VG00 as we do not have any other disks.

/dev/vg00/lv-private

What happens if I do not mirror it.

What happens if we just lvcreate, extendfs and mount it.

Obviously the free PEs in PV1 is less than the free PEs in PV2 as the lv-private is created in just PV1.

What are the implications, guidelines. Please suggest. Many thanks.

 

 # vgdisplay vg00
--- Volume groups ---
VG Name                     /dev/vg00
VG Write Access             read/write
VG Status                   available
Max LV                      255
Cur LV                      15
Open LV                     15
Max PV                      16
Cur PV                      2
Act PV                      2
Max PE per PV               4384
VGDA                        4
PE Size (Mbytes)            16
Total PE                    8748
Alloc PE                    4970
Free PE                     3778
Total PVG                   0
Total Spare PVs             0
Total Spare PVs in use      0

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5 REPLIES 5
Torsten.
Acclaimed Contributor

Re: what happens if non-root LV is not mirrored

It is gone if the disk fails. you need to restore your backup in this case.


Hope this helps!
Regards
Torsten.

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pooderbill
Valued Contributor

Re: what happens if non-root LV is not mirrored

So you created an LVOL that is larger than the remaining extents on one of the disks (PV). Since this LVOL overlaps into the second PV, you cannot mirror it. HP-UX is unaffected by this arrangement, and if you never have a disk failure on either disk, you'll be just fine.

The purpose of mirroring is to reduce the outage caused by one of the highest failure rate components in computer systems. Actually, anything that is mechanical and especially anything that rotates constantly (fans, disks) *will* fail. For your 2-disk vg00, a failure will still allow the system to keep running but the new LVOL will generate nasty errors, some of which may tend to lock up shell sessions and applications. But most important: You have now lost the data on that LVOL. The PV will have to be replaced, the previous LVOLs remirrored, then the new LVOL must be removed and recreated. Then you can restore the data from your backup tapes.

I would never put anything important on this LVOL...you could be down for days if you can't replace the disk or do not have a backup.

masood bt
Regular Advisor

Re: what happens if non-root LV is not mirrored

Thanks Torsten

 

 

no, the lvol is not that large. It is just that the one who created this lvol didnt realise he has to mirror it.

There is a lot of space in both the PVs of rootvg.

I think I got the answer. If the disk fails on which this lv-private is created then that lv is lost forever (if there is no backup)

 

I thought - there could be some performance issues because of more IO activities on just one PV. I now understand that is not the case.,.....

Behind every Great fortune, there is a crime
Bill Hassell
Honored Contributor

Re: what happens if non-root LV is not mirrored

There is no performance advantage for simple LVM mirroring. For reads, only one disk will be used, and writes will be slightly slower as both disks must be written.



Bill Hassell, sysadmin
Patrick Wallek
Honored Contributor

Re: what happens if non-root LV is not mirrored

If you have the space in VG00 mirror the LV now.  You don't HAVE to mirror only when creating an LV.  That can be done any time.

 

# lvextend -m 1 /dev/vg00/lv-private

 

Wait for the mirrors to sync, which may be a few minutes depending on the size of the LV, and you are done.  Now no more worries about that LV being destroyed if that disk fails.