Re: [PATCH] lvm: add lvm cache logical volume handling

[Michael Chang]

On Thu, Oct 24, 2019 at 05:21:04PM +0200, Daniel Kiper wrote:
> On Thu, Oct 24, 2019 at 04:43:58AM +0000, Michael Chang wrote:
> > On Wed, Oct 23, 2019 at 12:48:23PM +0200, Daniel Kiper wrote:
> > > On Wed, Oct 16, 2019 at 06:15:30AM +0000, Michael Chang wrote:
> > > > The lvm cache logical volume is the logical volume consisting of the
> > > > original and the cache pool logical volume. The original is usually on a
> > > > larger and slower storage device while the cache pool is on a smaller
> > > > and faster one. The performance of the original volume can be improved
> > > > by storing the frequently used data on the cache pool to utilize the
> > > > greater performance of faster device.
> > > >
> > > > The default cache mode "writethrough" ensures that any data written will
> > > > be stored both in the cache and on the origin LV, therefore grub can go
> > > > straight to read the original lv as no data loss is guarenteed.
> > > >
> > > > The second cache mode is "writeback", which delays writing from the
> > > > cache pool back to the origin LV to have increased performance. The
> > > > drawback is potential data loss if losing the associated cache device.
> > > >
> > > > During the boot time grub reads the LVM offline i.e. LVM volumes are not
> > > > activated and mounted, IMHO it should be fine to read directly from
> > > > original lv since all cached data should have been flushed back in the
> > > > process of taking it offline.
> > >
> > > Is it possible to enforce all GRUB writes to the original device instead
> > > of cache during installation process?
> >
> > I couldn't give concrete answer whether it is possible, but it sounds to
> > me not good idea because in general bypassing the cache could
> > potentially break the consistency of data and the data being cached.
> >
> > Perhaps some system calls could help to sync the data out of the lvm
> > cache to the original LV during installation process. It seems fsync
> > does it for us, but I didn't have good idea either if it is enough.
> 
> May I ask you to investigate that and if it is needed add required
> install code. I mean fsync(), etc.

Sure. And please bear with me if it takes long to answer as other task
was sidetracking. 

Thanks,
Michael

> 
> Daniel
> 
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