Re: [Question] why not flush device cache at _vg_commit_raw

[Anthony Iliopoulos <ailiop@suse.com>]

On Mon, Jan 22, 2024 at 01:48:41PM +0100, Zdenek Kabelac wrote:
> Dne 22. 01. 24 v 12:22 Su Yue napsal(a):
> > Hi lvm folks,
> >    Recently We received a report about the device cache issue after vgchange —deltag.
> > What confuses me is that lvm never calls fsync on block devices even at the end of commit phase.
> > 
> > IIRC, it’s common operations for userspace tools to call fsync/O_SYNC/O_DSYNC while writing
> > critical data. Yes, lvm2 opens devices with O_DIRECT if they support , but O_DIRECT doesn't
> > provide data was persistent to storage when write returns. The data can still be in the device cache,
> > If power failure happens in the timing, such critical metadata/data like vg metadata could be lost.
> > 
> > Is there any particular reason not to flush data cache at VG commit time?
> > 
> 
> Hi
> 
> It seems the call to 'dev_flush()' function got somehow lost over the time
> of conversion to async aio usage - I'll investigate.
> 
> On the other hand the chance here of losing any data this way would be
> really really very specific to some oddly behaving device.

There's no guarantee that data will be persisted to storage without
explicitly flushing the device data cache. Those are usually volatile
write-back caches, so the data aren't really protected against power
loss without fsyncing the blockdev.

Regards,
Anthony