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

[Naohiro Aota]

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.

Alright, I will change the code to do so.