On 28/06/2009 20:47, Leslie Rhorer wrote:
I have a few questions. Some RAID implementations will simply
refuse to create or grow an array if all the targets are not precisely
the
same size. Clearly this is not the case for mdadm. Not all drives of a
given "size" are actually precisely the same size, however, and I am
using
unpartitioned drives for my RAID systems. What happens if I add a drive
whose apparent physical size is a bit smaller than the device size used
to
create the array?
For RAID 4/5/6, I think it'll be refused.
Do you know if the refusal would include an error message clearly
indicating why the growth is refused?
I don't know about current versions, but I just tried it with loop
devices on CentOS 5 with mdadm 2.6.4, starting with a 3-member RAID-5
and trying to add a slightly-too-small 4th, and it's not a helpful error
message: "mdadm: add new device failed for /dev/loop4 as 3: Invalid
argument"
I may have to rethink my position on using raw drives. If I
partition the drives, I can make the partition a bit smaller than the whole
drive, allowing for the addition of a future drive whose size is a bit off.
I hate to waste space, but being stuck with an undersized or limping array
is worse.
Yes it is. You can still use raw devices, just give the --size argument
when creating your array, and it won't use the largest size possible.
You'll want to work out how many KiB (i.e. 2^10 bytes) there are in a
drive manufacturer's MB (10^6), GB (10^9) or TB (10^12) and use that as
appropriate. For example, presumably all manufacturers' 500G drives will
have at least 500*10^9 bytes of storage on them, and divided by 1024
that's 488281250.
Cheers,
John.
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