Extend LVOL with PVG-strict/distributed Allocation

[George_Dodds]

Hi all

Quick question, I need to extend an LV with a PVGstrict allocation, I'll be adding vdisks to the VG from an eva.

Question is do I need to add 4 vdisks/luns to be able to extend the lv and keep the PVGstrict allocation?

vgdisplay /dev/vg01
--- Volume groups ---
VG Name                     /dev/vg01
VG Write Access             read/write
VG Status                   available, exclusive
Max LV                      255
Cur LV                      10
Open LV                     10
Max PV                      16
Cur PV                      6
Act PV                      6
Max PE per PV               8000
VGDA                        12
PE Size (Mbytes)            32
Total PE                    24506
Alloc PE                    24471
Free PE                     35
Total PVG                   2
Total Spare PVs             0
Total Spare PVs in use      0


lvdisplay -v /dev/vg01/lvol1|more
--- Logical volumes ---
LV Name                     /dev/vg01/lvol1
VG Name                     /dev/vg01
LV Permission               read/write
LV Status                   available/syncd
Mirror copies               0
Consistency Recovery        MWC
Schedule                    parallel
LV Size (Mbytes)            81280
Current LE                  2540
Allocated PE                2540
Stripes                     0
Stripe Size (Kbytes)        0
Bad block                   NONE
Allocation                  PVG-strict/distributed
IO Timeout (Seconds)        default

   --- Distribution of logical volume ---
   PV Name                 LE on PV  PE on PV
   /dev/dsk/c14t1d6        296       296
   /dev/dsk/c16t1d4        960       960
   /dev/dsk/c10t1d5        295       295
   /dev/dsk/c12t1d3        989       989

[Patrick Wallek]

In a nutshell, yes.

As it stands, you can increase this LV by 295 PE's, 9,440 MB, without having to add more disks to the PVG.

Once you add the disks to the VG, don't forget to add them to the PVG as well.

[George_Dodds]

Hi Patrick, where are you getting the 295 free pe's from, as I only see 35 free pe's in the VG.

Cheers

George

[Patrick Wallek]

My apologies.  I misread the output you provided.  I saw the output for the distribution of PEs on the various disks and thought it was the number of PEs that were still FREE on those disks.

In thinking about this some more, I am not entirely sure that you will be able to add disks and grow the logical volume.

From the lvcreate man page, talking about the '-D' option for a distributed LV:

When the distributed allocation policy is turned on, only one free extent is allocated from the first available physical volume.  The next free extent is allocated from the next available physical volume.  Allocation of free extents proceeds in round-robin order on the list of available physical volumes.

When set up initially, say with 4 disks in a PVG, the LV will be created with one extents on PV-A, one on PV-B, one on PV-C, one on PV-D and then start with PVA again.  BUT, the thing I am not sure about is the behavior when you fill up the first 4 disks.  

If you add 4 more disks (PV-E thru PV-H) to the PVG, what I **THINK** will happen is that LVM will try to add extents on *ALL* disks in the PVG, PV-A thru PV-H.  I don't think it will use only the new disks.

I think you may be forced to disable the distributed option on this, or any other LVs, in order to expand them.

You can do that with the 'lvchange' command.  For example:

# lvchange -D n /dev/vg01/lvol01

If you were using traditional striping, then you could add an equal number of disks to the VG and the LV would use them.  Unfortunately I don't think this will work with the distributed LV.

[George_Dodds]

Cheers Patrick

I've put the question to hp support so will see what they come back with.

Not too keen on running an lvchange on this lv, so if I can keep it PVGstrict then better.