PE size

[Ngoh Chean Siung]

Hi,

1) Any different to hv bigger PE size if compared to small PE size?

2) After initialized the PV with PE size = 8MB, is it possible to modify the PE size?

regards.

[Robert-Jan Goossens]

Hi,

1) no

2) not pv but volume group, you can not change a volume group without recreating the volume group. Look at the -s option in the manaul page of vgcreate.

Best regards,
Robert-Jan

[Wim Rombauts]

1) Yes.

There is a limit in the number of PE's you can have in your VG. If you only have a few small disks, you can use a PE-size of 1MB. If you try that wen using 20 disks of 144GB, the VG-creation will fail.
If you use the distribution option, and you are writing to big files, you will have a better distribution of IO over your disks is your PE-size is small. Writing a 10MB file over 1MB PE's will use 10 disks in parallel. Writing the same file to 16MB PE's will send all IO to 1 disk.

2) No, unless you recreate your volume group.

[Robert-Jan Goossens]

Here is a good doc.

http://www4.itrc.hp.com/service/cki/docDisplay.do?docLocale=en_US&docId=200000068583821

Document description: LVM restrictions to adding a larger disk to a volume group
Document id: KBRC00000244

Regards,
Robert-Jan

[Ngoh Chean Siung]

Hi,

Just hv 1 part that not very clear.

Based on below statement, what is the LVM table spaces means?

Since the LVM table spaces are maintained in the disk headers (VGRA) and are static, the restricting values of Max PE per PV, PE Size,
and Max PV cannot be changed on an existing volume group.

regards.

[Steven E. Protter]

Since the LVM table spaces are maintained in the disk headers (VGRA) and are static, the restricting values of

>>>>
Max PE per PV, PE Size,
and Max PV cannot be changed on an existing volume group.
<<<<<<

These values are set up when you create the volume group with the vgcreate command. The only way to change them is to back up all the data, destroy the volume group, recreate the volume group and then restore the data.

Quite a pain.

PE Size does not evidently have much impact on say database performance. I did a thread on that question a while back.

SEP

[Ngoh Chean Siung]

Hi,

Sorry for the confusing question.

What I want to know is what is LVM table spaces? What does it consist of?

regards.

[Con O'Kelly]

Hi

Have a look at Chp 16 - LVM (Software recovery Handbook) which I have attached.
This gives an explanation on LVM Structural Information.

LVM tablespaces refers to the structural information that LVM stores on each disk that is initialized in LVM. ie any disk that is used in LVM conatins an LVM header.
See the attachment for more details.

Cheers
Con

[Sridhar Bhaskarla · Accepted solution]

Hi,

LVM table space consists of PVRA - PV specific information , VGRA - VG specific information and BDRA - Boot specific information in case of boot disks. This information is written into one extent on each PV.

By default PE size is 4 MB. On larger VGs (having larger disks hence more number of PEs, more number of PVs etc.,), 4 MB may not be sufficient to hold this configuration data. So, PE size is to be increased as required. Increasing PE size will have no effect on the performance. However, it can only waste atleast (PE-1)MB of disk space. For ex., say you have exactly 63MB (hypothetical) disk. If you add it to a VG with 4MB of PE size, you will get 15 extents and 3 MB will be wasted. However, if the PE size is 32MB, you will get only 1 extent with 31 MB wasted. More same sized PVs means more space wasted. However on disks of multiple GB in size, one can afford it.

-Sri

[Dietmar Konermann]

The LVM chappter attached earlier in this thread is out-dated. More details can be found in the recent copy of the Software Recovery Handbook (see 'useful links' at the bottom of the HP-UX forum page).

You may also have a look at the lvmcompute tool I attached. Use -h to get the synopsis.

Best regards...
Dietmar.

[Ngoh Chean Siung]

Hi Dietmar,

Can you provide me the link?

regards.

[Ngoh Chean Siung]

Hi,

On my Server A,

--- Volume groups ---
VG Name /dev/vg00
VG Write Access read/write
VG Status available
Max LV 255
Cur LV 14
Open LV 14
Max PV 16
Cur PV 1
Act PV 1
Max PE per PV 4384
VGDA 2
PE Size (Mbytes) 16
Total PE 4374
Alloc PE 797
Free PE 3577
Total PVG 0
Total Spare PVs 0
Total Spare PVs in use 0

On my server B,

--- Volume groups ---
VG Name /dev/vg01
VG Write Access read/write
VG Status available
Max LV 255
Cur LV 10
Open LV 10
Max PV 16
Cur PV 1
Act PV 1
Max PE per PV 8683
VGDA 2
PE Size (Mbytes) 4
Total PE 8681
Alloc PE 2641
Free PE 6040
Total PVG 0
Total Spare PVs 0
Total Spare PVs in use 0

1) For Server A, why Max PE per PV is 4384 and Total PE is 4374? (10 PE missing?)

2) For Server B, why Max PE per PV is 8683 and Total PE is 8681? (only 2 PE missing?)

regards.

[Dietmar Konermann]

Here we are...

http://itrc.hp.com/service/iv/node.do?node=prod/WW_Start/N1|16

Please note that access to this doc is tied to a valid support contract.

Ciao...
Dietmar.

[Ngoh Chean Siung]

Hi Dietmar,

Thanks.

My colleague has linked support agreement. How I can share with him? Can I hv the steps to do it?

regards.