On 09/20/2010 05:51 PM, Anthony Liguori wrote:
On 09/20/2010 10:08 AM, Kevin Wolf wrote:
If you're comfortable with a writeback cache for metadata, then you
should also be comfortable with a writeback cache for data in which
case, cache=writeback is the answer.
Well, there is a difference: We don't pollute the host page cache with
guest data and we don't get a virtual "disk cache" as big as the host
RAM, but only a very limited queue of metadata.
Would it be a mortal sin to open the file twice and have a cache=none
version for data and cache=writeback for metadata?
The two definitely aren't consistent with each other but I think the
whole point here is that we don't care.
It opens up some other possibilities too like cache=none for data and
cache=writethrough for metadata which may be a useful combination.
I've thought of this (and I think perhaps suggested it on this list).
The question is whether the kernel doesn't slow direct io when page
cache is present for the file (but in unconflicting ranges).
I think it's considered a valid use case (backing up a database file
while the database is O_DIRECTing into it) but I don't know if the code
was actually updated to support this.
--
error compiling committee.c: too many arguments to function
