On Thu, 16 Feb 2012, Jeremy Chadwick wrote:
On Thu, Feb 16, 2012 at 06:34:53PM -0700, Warren Block wrote:
(...Linux mdadm)
So for version 0.90 of their metadata format, you lose drive capacity by
about 64-128KBytes, given that the space is needed for metadata. For
version 1.0, I'm not sure. For version 1.1 it looks like the metadata
can be stored at the beginning.
So overall, this sounds to me like the equivalent of if GEOM was to
"lie" about the actual capacities of the devices when using classes that
require use of metadata (gmirror, etc.).
Sorry, I may be misunderstanding your point. GEOM classes don't lie,
they accurately represent the space. The space provided by a gmirror is
one block less than the actual space occupied, to allow for the metadata
block at the end. The problem is that GPT puts backup partition tables
at the end of the physical (not logical) device. Create a GEOM device on
that drive, and the GEOM metadata overwrites the backup GPT partition
table. Well, the last block of it, anyway.
But create the GEOM device inside a GPT partition that spans the drive,
and things are fine. The GPT backup tables are safely outside the GEOM
metadata, which is safely outside of the data.
Short-form: GPT tables are at the absolute start and end of the physical
disk. GEOM metadata is relative, at the end of the logical device, not
necessarily the end of the physical device.
On the other hand, GEOM stuff works inside GPT partitions. And if
that's not acceptable, MBR partitions will be around for a long
time.
MBR partitions don't scale past 2TB. Arguing that use of MBR is an
acceptable workaround is the equivalent to burying one's head in the
sand. Let's try to accept the future, not feign ignorance.
I wasn't recommending it. If putting GEOM data inside GPT partitions
isn't acceptable (but why not?), there's the alternative of not using
any partitioning at all. Create the GEOM device on the bare drive using
the full space. Of course it won't boot...
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