[Qemu-devel] Re: [PATCH] Add cache=volatile parameter to -drive

[Alexander Graf]

On 17.05.2010, at 14:58, Anthony Liguori wrote:

> On 05/17/2010 05:14 AM, Alexander Graf wrote:
>> Usually the guest can tell the host to flush data to disk. In some cases we
>> don't want to flush though, but try to keep everything in cache.
>> 
>> So let's add a new cache value to -drive that allows us to set the cache
>> policy to most aggressive, disabling flushes. We call this mode "volatile",
>> as guest data is not guaranteed to survive host crashes anymore.
>> 
>> This patch also adds a noop function for aio, so we can do nothing in AIO
>> fashion.
>> 
>> Signed-off-by: Alexander Graf<address@hidden>
>>   
> 
> I'd like to see some performance data with at least an ext3 host file system 
> and an ext4 file system.

For ext3 data, please see my cover-letter from v2:

http://www.mail-archive.com/address@hidden/msg31714.html

> My concern is that ext3 exaggerates the cost of fsync() which will result in 
> diminishing value over time for this feature as people move to ext4/btrfs.

There will be ext3 file systems for years out. Just because people can use 
better and faster file systems doesn't mean they do. And I'm sure they can't 
always choose. If anything, I can try and see what the numbers look like for 
xfs.


Alex