Re: [PATCH 09/19] btrfs: limit super block locations in HMZONED mode

[David Sterba]

On Tue, Jun 18, 2019 at 09:01:35AM +0000, Naohiro Aota wrote:
> On 2019/06/18 7:53, David Sterba wrote:
> > On Fri, Jun 07, 2019 at 10:10:15PM +0900, Naohiro Aota wrote:
> >> When in HMZONED mode, make sure that device super blocks are located in
> >> randomly writable zones of zoned block devices. That is, do not write super
> >> blocks in sequential write required zones of host-managed zoned block
> >> devices as update would not be possible.
> > 
> > This could be explained in more detail. My understanding is that the 1st
> > and 2nd copy superblocks is skipped at write time but the zone
> > containing the superblocks is not excluded from allocations. Ie. regular
> > data can appear in place where the superblocks would exist on
> > non-hmzoned filesystem. Is that correct?
> 
> Correct. You can see regular data stored at usually SB location on HMZONED fs.
> 
> > The other option is to completely exclude the zone that contains the
> > superblock copies.
> > 
> > primary sb			 64K
> > 1st copy			 64M
> > 2nd copy			256G
> > 
> > Depends on the drives, but I think the size of the random write zone
> > will very often cover primary and 1st copy. So there's at least some
> > backup copy.
> > 
> > The 2nd copy will be in the sequential-only zone, so the whole zone
> > needs to be excluded in exclude_super_stripes. But it's not, so this
> > means data can go there.  I think the zone should be left empty.
> > 
> 
> I see. That's more safe for the older kernel/userland, right? By keeping that zone empty,
> we can avoid old ones to mis-interpret data to be SB.

That's not only for older kernels, the superblock locations are known
and the contents should not depend on the type of device on which it was
created. This can be considered part of the on-disk format.