WAPBL and "cache flush not supported by backend"

[Luke Maurits <luke%maurits.id.au@localhost>]

Hi list,

I see "xbd0: WARNING: cache flush not supported by backend" in dmesg
whenever my NetBSD 6 STABLE Xen VPS boots.  I assume this is something
to do with the configuration of Dom0, which I have no control over.

Despite this warning, everything *seems* to work fine.  Things like
untarring a pkgsrc branch go about 10 times faster on a file system
mounted with -o log than on one without.  After a crash I see "/:
replaying log to disk", with no apparent errors.

Is this warning safe to ignore?  Does the lack of cache flushing just
lead to something like reduced performance, or is there a risk of
Really Bad Stuff happening under certain conditions (like file system
corruption)?

Cheers,
Luke