topic Re: PE size in Operating System - HP-UX

PE size

Ngoh Chean Siung — Mon, 22 Nov 2004 03:03:01 GMT

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.

Re: PE size

Robert-Jan Goossens — Mon, 22 Nov 2004 03:11:04 GMT

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

Re: PE size

Wim Rombauts — Mon, 22 Nov 2004 04:22:20 GMT

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.

Re: PE size

Robert-Jan Goossens — Mon, 22 Nov 2004 04:48:29 GMT

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

Re: PE size

Ngoh Chean Siung — Mon, 22 Nov 2004 20:52:36 GMT

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.

Re: PE size

Steven E. Protter — Mon, 22 Nov 2004 21:24:35 GMT

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

Re: PE size

Ngoh Chean Siung — Mon, 22 Nov 2004 21:35:16 GMT

Hi,

Sorry for the confusing question.

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

regards.

Re: PE size

Con O'Kelly — Mon, 22 Nov 2004 21:49:57 GMT

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

Re: PE size

Sridhar Bhaskarla — Tue, 23 Nov 2004 02:54:39 GMT

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

Re: PE size

Dietmar Konermann — Tue, 23 Nov 2004 03:30:13 GMT

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.

Re: PE size

Ngoh Chean Siung — Tue, 23 Nov 2004 03:55:38 GMT

Hi Dietmar,

Can you provide me the link?

regards.

Re: PE size

Ngoh Chean Siung — Tue, 23 Nov 2004 04:05:17 GMT

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.

Re: PE size

Dietmar Konermann — Tue, 23 Nov 2004 04:05:18 GMT

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.

Re: PE size

Ngoh Chean Siung — Tue, 23 Nov 2004 04:58:34 GMT

Hi Dietmar,

Thanks.

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

regards.