Re: Shouldn't backend devices for VMX domain disks be opened with O_DIRECT?

[Anthony Liguori <aliguori@us.ibm.com>]

Steve Dobbelstein wrote:

>While running some disk performance tests for VMX domains we noticed that
>writes to the backend device for a VMX domain's disk go through the buffer
>cache, that is, they are not written immediately to disk.  Shouldn't the
>I/Os go straight to the backend device, i.e., the device should be opened
>with O_DIRECT or some such?  From the domain's perspective it expects the
>data to be physically on the device, but in reality it is not.   There are
>things, such a writes to a file system journal, that the OS in the domain
>will expect to be on disk.  If the whole system crashes before the buffer
>cache in dom0 is written to disk, those writes may not be on the disk.
>When the domain is started again it may find the file system in an
>inconsistent state, due to writes to the journal that didn't make it to
>disk, and may not be able to recover.
>
>It seems to me that if a domain expects things to be physically on its
>frontend device that they should be physically on the backend device as
>well.  Or am I missing something from the bigger picture?
>  
>
I would doubt it.  Since it's usually opening a file, and qemu-dm is 
emulating a contigous disk, you probably want the buffer cache to 
reorder events.

Are you seeing a performance improvement?  Should be easy to check.

Regards,

Anthony Liguori

>Steve D.
>
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