Re: WAPBL and "cache flush not supported by backend"

[Manuel Bouyer <bouyer%antioche.eu.org@localhost>]

On Sun, Feb 17, 2013 at 05:58:59PM -0800, Luke Maurits wrote:
> 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)?

The warning is there for a reason. In theory, this issue could lead
to filesystem corruption when a poweroff occurs with data in
the disk or dom0 cache (if dom0 is doing write back) that are not yet on
platters (WAPBL use cache flush as a barrier, to make sure all data before
the upcoming writes are on disk and things cannot be reordered).
Whenever this is a real problem depends on dom0 setup/behavior, and hardware
(the dom0 could turn off the write back disk cache, or the storage
could be a RAID controller with battery-backed cache).

-- 
Manuel Bouyer <bouyer%antioche.eu.org@localhost>
     NetBSD: 26 ans d'experience feront toujours la difference
--