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List:       evms-devel
Subject:    Re: [Evms-devel] maximum size for storage containers...
From:       Kevin Corry <corryk () us ! ibm ! com>
Date:       2003-03-18 15:15:33
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Hi Brian,

On Friday 14 March 2003 09:21, Brian Meifert wrote:
> I know that under linux there is a 2 TB limit on the size of logical
> volume, but is there a limit on the maximum amount of storage, size wise,
> that can be put into a single storage container? and if so what is that
> limit?

On 2.4 kernels, there is a 2 TB device-size limit. On 2.5 kernels, this limit 
should go away (or rather, increase to 8 zettabytes - more than enough 
storage to get us by for a few years).

An LVM container can consume up to 255 objects (PVs). Each PV is logically 
divided into fixed-size chunks called extents (PEs). The total number of PEs 
in the container is stored as a 32-bit number in the metadata, so you can 
only have 2^32 (~4 billion) PEs. The default PE size when you create a 
container is 16 MB, but it can be increased to 16 GB (provided all PVs are at 
least 80 GB - little known bit of LVM trivia: each consumed PV must be at 
least 5 times the PE size). Thus, with the default PE size, the container can 
consume 64 petabytes, and with the max PE size, it can consume 64 exabytes. 
Since containers are actually devices, these limits should not be affected by 
the kernel version.

However, keep in mind also, that each region created from a container can 
only have up to 2^16 (~64 thousand) extents assigned to it (due to 16-bit 
indexing in the metadata). So with the default PE size of 16 MB, your max 
volume size will be 1 TB. If you want volumes larger than 1 TB, be sure to 
set your PE size appropriately when you create the container, since it cannot 
be changed after the container is created.

Hope this helps.

-- 
Kevin Corry
corryk@us.ibm.com
http://evms.sourceforge.net/


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