Il 24-02-2018 23:07 Dave Chinner ha scritto:

Define "busy file", please.

Think about a running virtual machine. Maybe an XFS-based virtual image 
(ie: a CentOS 7 guest).

If the file is being actively written, then the clone will not be
consistent.

Yes, it's just like any other snapshot process - you have to quiesce
everything that is writing to the file before cloning it. i.e. the
data in the file needs to be in a stable, consistent, unchanging
state if you want the clone to contain consistent data...

About *what* level of consistency are we speaking? I understand that 
application-level consistency requires a quiesced filesystem and, 
possibly, an application-level agent. But is it a quiesced filesystem a 
requisite for a *crash-consistent* ie: pull the plug) snapshot?

In other words: would a cp --reflink=always <vmdisk> <snapshot> of a 
runnig virtual machine produce an usable, crash-consistent snapshot, or 
it risks ending with binary garbage?

Thanks.

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Danti Gionatan
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