[dm-devel] Performance Comparison among EnhanceIO, bcache and dm-cache.

[Joe Thornber]

On Tue, Jun 11, 2013 at 03:05:07PM +0000, OS Engineering wrote:

...

> Dm-cache commits on-disk metadata every time a REQ_SYNC or REQ_FUA
> bio is written. If no such requests are made then it commits
> metadata once every second. If power is lost, it may lose some
> recent writes.

Not true (though it is true for thinp, which may be where you got this
idea?).  For caching we have to commit whenever data is moved about,
otherwise a crash could result in us reading data that is not just out
of date (acceptable for some), but used to belong to a totally
different part of the device (always unacceptable).

- Joe