Re: [Qemu-devel] [RFC] Disk integrity in QEMU

[Avi Kivity <avi@redhat.com>]

Anthony Liguori wrote:

[O_DSYNC, O_DIRECT, and 0]

>
> Thoughts?

There are (at least) three usage models for qemu:

- OS development tool
- casual or client-side virtualization
- server partitioning

The last two uses are almost always in conjunction with a hypervisor.

When using qemu as an OS development tool, data integrity is not very 
important.  On the other hand, performance and caching are, especially 
as the guest is likely to be restarted multiple times so the guest page 
cache is of limited value.  For this use model the current default 
(write back cache) is fine.

The 'causal virtualization' use is when the user has a full native 
desktop, and is also running another operating system.  In this case, 
the host page cache is likely to be larger than the guest page cache.  
Data integrity is important, so write-back is out of the picture.  I 
guess for this use case O_DSYNC is preferred though O_DIRECT might not 
be significantly slower for long-running guests.  This is because reads 
are unlikely to be cached and writes will not benefit much from the host 
pagecache.

For server partitioning, data integrity and performance are critical.  
The host page cache is significantly smaller than the guest page cache; 
if you have spare memory, give it to your guests.  O_DIRECT is 
practically mandataed here; the host page cache does nothing except to 
impose an additional copy.

Given the rather small difference between O_DSYNC and O_DIRECT, I favor 
not adding O_DSYNC as it will add only marginal value.

Regarding choosing the default value, I think we should change the 
default to be safe, that is O_DIRECT.  If that is regarded as too 
radical, the default should be O_DSYNC with options to change it to 
O_DIRECT or writeback.  Note that some disk formats will need updating 
like qcow2 if they are not to have abyssal performance.

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