Re: [PATCH][RF C/T/D] Unmapped page cache control - via boot parameter

[Avi Kivity <avi@redhat.com>]

On 03/16/2010 12:26 PM, Christoph Hellwig wrote:
> Avi,
>
> cache=writeback can be faster than cache=none for the same reasons
> a disk cache speeds up access.  As long as the I/O mix contains more
> asynchronous then synchronous writes it allows the host to do much
> more reordering, only limited by the cache size (which can be quite
> huge when using the host pagecache) and the amount of cache flushes
> coming from the host.  If you have a fsync heavy workload or metadata
> operation with a filesystem like the current XFS you will get lots
> of cache flushes that make the use of the additional cache limits.
>    

Are you talking about direct volume access or qcow2?

For direct volume access, I still don't get it.  The number of barriers 
issues by the host must equal (or exceed, but that's pointless) the 
number of barriers issued by the guest.  cache=writeback allows the host 
to reorder writes, but so does cache=none.  Where does the difference 
come from?

Put it another way.  In an unvirtualized environment, if you implement a 
write cache in a storage driver (not device), and sync it on a barrier 
request, would you expect to see a performance improvement?


> If you don't have a of lot of cache flushes, e.g. due to dumb
> applications that do not issue fsync, or even run ext3 in it's default
> mode never issues cache flushes the benefit will be enormous, but the
> data loss and possible corruption will be enormous.
>    

Shouldn't the host never issue cache flushes in this case? (for direct 
volume access; qcow2 still needs flushes for metadata integrity).

> But even for something like btrfs that does provide data integrity
> but issues cache flushes fairly effeciently data=writeback may
> provide a quite nice speedup, especially if using multiple guest
> accessing the same spindle(s).
>
> But I wouldn't be surprised if IBM's exteme differences are indeed due
> to the extremly unsafe ext3 default behaviour.
>    


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