concurrent direct IO write in xfs

[Zheng Da]

Hello,

On Mon, Jan 23, 2012 at 7:34 PM, Stan Hoeppner <stan at hardwarefreak.com>wrote:

> On 1/23/2012 2:51 PM, Zheng Da wrote:
>
> > tmpfs doesn't support direct IO.
>
> Of course not.  tmpfs resides entirely within the page cache (or some of
> it in swap).  The whole point of direct IO is to bypass the page cache,
> transferring data directly between user space memory and the storage
> device.  As tmpfs is built entirely within the page cache, direct IO is
> obviously impossible.  And it's also obviously unnecessary.
>
> Yes, you will need to rewrite your application to use tmpfs as direct IO
> calls won't work.  This is something you obviously would rather not do.
>  Which brings us back to Dave's question, which you have not answered:
>
> What exactly is the purpose of your program?  What does it aim to
> accomplish?  Is it for a database application?  A word processor?  Or
> simply a filesystem tester?  What do _you_ aim to accomplish with this
> programming effort?
>
> I'm trying to test the scalability of page cache in the random access
workload. And the cache hit rate is also relatively low.
The cache is now implemented in the user space. When cache misses, I need
to read data from the file system with direct IO.

Da
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