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Re: [dm-devel] dd between thin provisioned devices



thanks for the reply. What i really wanna to ask is thin clone/mirror supported or not.
 
I noticed the time later SCSI commad GET LBA STATUS can fullfil this requiremnt .
 
Before block copy, send GET LBA STATUS to check the corresponding blocks mapped or not.
if not, just skip copy. 


On Mon, Jan 28, 2013 at 5:06 PM, Zdenek Kabelac <zdenek kabelac gmail com> wrote:
Dne 27.1.2013 11:01, devicemapper niko napsal(a):

Hi,everyone
if we dd between thin provisioned devices,
can we just get allocated blocks copied ?

Nope - devices mapper target operates on block level - not on the filesystem
level - so it has no knowledge about file layout.

What I could eventually imagine is some kind of block compactification (just like kernel does for hugepages) - but it's currently not being developed....



Will the output thin device get fully provisioned?
I glanced through the dm-thin code,
it turns out unmapped blocks return zeroed,am i right?

depends on settings - you could have either zeroed block or block
with content left on the disk - i.e. running thin with 1MB block/chunk size
means - before the first use whole 1MB must be zeroed - so it has its price.


Is there a trick to tell upper applications that all u got are zeroes.
or should they scan zeroed blocks themselves to trigger WRITE_SAME to reduce
I/O load?


Documentation/device-mapper/thin-provisioning.txt
see flag: skip_block_zeroing

Eventually for lvm2   lvcreate --zero y|n for thin pool creation.

Zdenek



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