Re: [ANNOUNCE] Native Linux KVM tool v2

From: Jeff Garzik
Date: Fri Jun 17 2011 - 03:22:09 EST

On 06/16/2011 07:22 AM, Ingo Molnar wrote:

* Christoph Hellwig<hch@xxxxxxxxxxxxx> wrote:

On Thu, Jun 16, 2011 at 12:57:36PM +0300, Pekka Enberg wrote:
Uh-oh. Someone needs to apply this patch to sync_file_range():

There actually are a few cases where using it makes sense. [...]

Such as? I don't think apps can actually know whether disk blocks
have been 'instantiated' by a particular filesystem or not, so the

Some details
None of these operations write out the fileâs metadata. Therefore, unless the appli-
cation is strictly performing overwrites of already-instantiated disk blocks, there
are no guarantees that the data will be available after a crash.

is rather misleading. This is a dangerous (and rather pointless)
syscall and this should be made much clearer in the manpage.

Not pointless at all -- see Linus's sync_file_range() examples in "Re: Unexpected splice "always copy" behavior observed" thread from May 2010.

Apps like MythTV may use it for streaming data to disk, basically shoving the VM out of the way to give the app more fine-grained writeout control.

Just don't mistake sync_file_range() for a data integrity syscall.


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