Re: dd between thin provisioned devices

[Zdenek Kabelac <zdenek.kabelac@gmail.com>]

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