vgcreate -s option

[Jim Tropiano_1]

Is there some sort of formula to use to know when to expand the # of physical _extent _size when doing a vgcreate.
or how large to make it 2,4,8,16,32,64,128.

[Devender Khatana · Accepted solution]

Hi,

The formula is listed in the attachment.

HTH,
Devender

[A. Clay Stephenson]

Essentially all you need to be concerned about is that the product of pe_size and max_pe is able to accomodate the largest physical drive/LUN that you ever anticipate
to be part of this VG. Smaller PE's mean that less of the last chunk of a drive is wasted if the size is not an exact multiple of PE size but that's a trivial concern. With one very dubious exception, PE size has no impact on performance. The exception is extent-based disk striping but here even the smallest possible PE size (1MB) is still too large to be be a good stripe chunk.

[Marvin Strong]

As Mr. Stephenson, says base it on what you think your largest lun will ever be.

Personally I usually allow for a lun twice the size of the current luns, as disk size continues to grow, I can see a day when I will be using luns larger than 35GB. And since I prefer to migrate to new disks using mirroring as its the easiest and normally requires no downtime. Allowing a lun size of double my current luns should allow the volume group to be valid for a long time to come. And allow for easy migrations in the future. At least until my lun size gets so large I need to recreate the vg for them.

[Tim Nelson]

keep in mind also that the extent size needs to be able to hold the entire mapping for the disk in the first reserverd PE. Larger disks have more PE and 1MB is not big enough.

Try to set to 1MB on a 36gb or larger disk. You should get an error. Unless something has changed in the last couple years this should hold true.

[Steven E. Protter]

Keep in mind, you can also improve your future capacity with the -p option in vgcreate.

If you spread the available extents over less than the default 255 possible disks, this helps out without using the -s option.

vgcreate ... -p 20

But you better not ever need more than 20 physical volumes in that scenario.

SEP

[Devender Khatana]

Hi,

->Keep in mind, you can also improve your future capacity with the -p option in vgcreate.

SEP -

The value of -s is important for new big size disks and can not be replaced by -p option. -p defines the max. no. of physical volumes configurable in a VG whereas -s option alongwith -e (Max_PE_PER_PV)option defines the maximum capacity you can configure for one physical disk. As the -e (MAX_PE_PER_PV) is limited with small PE_SIZE and can not address full capacity of nowdays available 36/72/146 GB drives you need to increase the default PE_Size with -s option.

-e options is set by default as per the largest physical volume while creating the LV. If you plan to add big LUN/Disks in the VG than of those on which VG in initially created then you should also set MAX_PE_PER_PV to a higher value using -e option while creating the VG.

These will the symptoms as mentioned in my earlier attachment as well.

http://forums1.itrc.hp.com/service/forums/questionanswer.do?threadId=739558

HTH,
Devender

[Cem Tugrul]

Jim,
i always use as like;
vgcreate -s 32 -e 25000 /dev/vgname /dev/dsk/c#t#d#

and up to know not seen any problem!!!

it is just my choice.

Good Luck,