Max Physical Extents (again)

[Tony Horton]

Hi,

I've searched and pe size comes up a lot but I haven't found anything much on Max physical extents. I'm just installing a new box, and want to avoid problems in future if I add new larger disks, The install is defaulting to max physical extents 4350 with pe size 8MB. (36GB root disk).

Even if I was to make the pe size 32 or 64MB I'm still going to be pretty limited as to how big a disk I can add later if I don't change max pysical extents.

Should I just make it 65535? I'm assuming once it's set its like pe size and not modifiable, Is there any downside to making it as big as it can be. say I decided the max disk I would want to add was 320 GB (hypothetically) should I just make it 32MB pe size and max phys extent 10,000?

Regards,

Tony.

PS. It's highly unlikely I will add any more disks to the root VG anyway but I will be setting up other VG's where it will be more relevant.

[A. Clay Stephenson · Accepted solution]

The typical way to handle this is to use the largest disk(s) while doing the vgcreate so that the issue is handled implicitly. You are correct that one set the number of PE's is fixed for all physical volumes (disks or array LUN's) that comprise the VG. There is a small penalty in intentionally oversizing the number of PE's and that is that kernal data space will increase. The size of the maps that do the actual translation from logical block numbers to physical block numbers is enlarged and wasted. The best policy is to set the PE size and man PE's to the largest anticipated requirement.

[Tony Horton]

Thanks Clay.

Looks like I was on the right track. The biggest disks I currently have are 73GB, I may need to add some bigger disks in the life of the server, maybe not, so I think I'll opt for a middle of the road approach and maybe allow for a max disk size of 300GB,

On the root volume group I may just allow for up to 73GB disks, I can't see myself needing a 73GB root disk, but better to be safe than sorry :)

Regards,

Tony.

[Suresh Patoria]

Hi,

u can define logical extend size while creating volume group use the -e option u can define maximum 32MB size

Thanx

[Steven E. Protter]

You can further increase the capacity of a volu me group with the -s option

The default size is 4 MB per extent. The extent size can be as big as 256 MB.

SEP

[Tony Horton]

Thanks guys was aware of the switches, just wasn't sure whether to take the approach of make it big as possible or make a decision on what was the biggest I was likely to need and set it accordingly.

Regards,

Tony.

[Srikanth Arunachalam]

Hi Tony,

I have a question on the same thread. Are the physical extend size propotional to that of size of logical volume. For example, if the volume is 75gb, should I just mention 8m and for 150gb, should i specify as 16m.

My largest volume group is of size 500GB.

Thanks,
Srikanth

[A. Clay Stephenson]

The total size of a VG doesn't matter; you simply need to choose a max_pe (with a maximum of 64Ki) and a pe_size so that the product of these will accomodate the largest anticipated physical volume (disk, LUN) in the volume group.

[Srikanth Arunachalam]

Hi Clay,

Yes accepted on the point. Are there any advantage of having lesser physical extend size, so that the extend size is efficiently utilized. If so, I will be happy to have 4m (minimum size) for all volumes.

On the other hand, if the maximum extends are quiet higher..does it takes longer time to scan and give the output and hence impact on I/O.

[A. Clay Stephenson]

No, with one very dubious exception, PE size has nothing to do with performance. That exception is extent-based striping but even in this case the smallest possible PE (1MiB) is still too large to be a good stripe size -- so it doesn't matter. PE size is really nothing more than kernel bookkeeping.

The only downside to a larger PE is that you might lose a small amount of disk space because you are dividing a finite disk size into larger chunks so that the disk size might not be an exact multiple of some larger PE but would be an exact multiple of a smaller PE.