[Cluster-devel] [GFS2 PATCH] GFS2: Eliminate bitmap clones

[Steven Whitehouse <swhiteho@redhat.com>]

Hi,


On 03/07/18 14:28, Bob Peterson wrote:
> Hi Steve,
>
> ----- Original Message -----
>>> Do we really still need "clone bitmaps" in gfs2? If so, why?
>>> I think maybe we can get rid of them. Can someone (Steve Whitehouse
>>> perhaps?) think of a scenario in which they're still needed? If so,
>>> please elaborate and give an example.
> (snip)
>> You need to ensure that the blocks cannot be reused in the same
>> transaction (thats true of all metadata blocks, not just inodes) in
>> order that recovery will work correctly. You cannot just eliminate the
>> bitmaps without adding a mechanism to prevent this reuse,
>>
>> Steve.
> I don't see how it's possible for a transaction to reuse the same blocks,
> even when transactions are combined.
Transactions get combined anyway as we batch them up for commit to the 
journal. The important thing is that we do not have blocks that get used 
in more than one way (data or metadata) in any one journal transaction. 
Otherwise when the journal is recovered we cannot figure out what the 
correct state should be, because we are missing information on the history.

So if we decide to find another mechanism to deal with that issue, then 
thats ok, but the usual caveats about backward compatibility apply of 
course, so it will not be easy to do. Also, we already have a todo list 
item to allow transactions to continue in parallel with the journal 
commit, and we should probably look at that problem first since it is is 
not easy and we don't want to do anything to make it harder than it 
needs to be in the mean time,

Steve.