contiguous PE allocation

[Mark Henry_1]

Hi All,

In a non-boot disk scenario, is the any advantage using strict PE allocation? Perhaps speed - there's no need to go to random locations for the PE's, but the pitfall of having no room to grow if there's an adjacent lvol is probably worse..?

Thanks,

Mark

[Mark Henry_1]

Oops, bad use of 'strict' - not talking about mirrors here..
-M

[A. Clay Stephenson]

"Strict" means that lvols cannot be mirrored on the same disk - that's a very good thing but you seem to be referring to contigiuos allocation which restricts all the PE's of a given LVOL mirror to be on the same volume and contigiously allocated. As you mention, those volumes cannot be extended later. While it's possible that there might be some advantage if doing purely sequential I/O that is such a rarity in the UNIX world as to be almost non-existent.

The main advantage of contigious LVOL's is that it makes things simple for booting and swapping especially while the box is coming up and is thus real dumb.

[Uday_S_Ankolekar]

Strict Allocation:
The purpose of disk mirroring is to have the data stored on more than one disk
in case of a disk failure. The lvcreate command sets the "strict" allocation
policy by default. This means that mirrors of a logical extent cannot share
the same physical volume.


Contiguous allocation ensures that physical extents are allocated in ascending
order without any gap between adjacent extents and all extents are contained in
a single physical volume.

The contiguous allocation policy is turned off by default when creating a
logical volume. It can be turned on with the "-C y" option with the lvcreate
lvchange commands.
-USA..

[James R. Ferguson]

Hi Mark:

'Contiguous' allocation means physical extents are allocated in ascending order; no gap is allowed between physical extents within a mirror copy; and physical extents of any mirror copy all reside on a single physical volume.

'Strict' allocation means that mirror copies of logical extents cannot be allocated to share the same physical volume or physical volume group as the mirror copy.

Thus, whether or not this a boot disk, I definitely want my mirror copies to reside on different physical volumes. That is, I definitely want to set 'strict'.

As for 'contiguous' alllocation setting it means that expanding a logical volume becomes more difficult (although it can be done). I don't normally set 'contiguous', and I don't worry about head movement on a disk device within the physical space assigned to a logical volume. I suppose if one were doing large sequential reads or writes some loss of performance could be measured for non-contiguous LVs, but there are probably better improvements to be made elsewhere (e.g. in application code).

Regards!

...JRF...

[James Odak]

when adding extra dump space (HP techs usually need a full dump to analyze system TOC's and default installs never make enough dump space) it has to be contiguous

other then that i've never heard of any other reason to use it.