On 2020/9/21 22:00, Christoph Hellwig wrote:

On Mon, Sep 21, 2020 at 05:54:59PM +0800, Coly Li wrote:

quoted

I am not sure whether virtual bcache device's optimal request size can
be simply set like this.

Most of time inherit backing device's optimal request size is fine, but
there are two exceptions,
- Read request hits on cache device
- User sets sequential_cuttoff as 0, all writing may go into cache
device firstly.
For the above two conditions, all I/Os goes into cache device, using
optimal request size of backing device might be improper.

Just a guess, is it OK to set the optimal request size of the virtual
bcache device as the least common multiple of cache device's and backing
device's optimal request sizes ?

Well, if the optimal I/O size is wrong, the read ahead size also is
wrong.  Can we just drop the setting?

I feel this is something should be fixed. Indeed I overlooked it until
you point out the issue now.

The optimal request size and read ahead pages hint are necessary, but
current initialization is simple. A better way might be dynamically
setting them depends on the cache mode and some special configuration.

By your inspiration, I want to ACK your original patch although it
doesn't work fine for all condition. Then we may know these two settings
(ra_pages and queue_io_opt) should be improved for more situations. At
lease for most part of the situations they provide proper hints.

How do you think of the above idea ?

Coly Li