>>> I think I can reduce data on the "full" bricks, solving the problem 
>>> temporarily.
>>> 
>>> The thing is, that the behavior changed from 3.12 to 6.5:   3.12 didn't 
>>> have problems with almost full bricks, so I thought everything was fine. 
>>> Then, after the upgrade, I ran into this problem. This might be a corner 
>>> case that will go away once no-one uses 3.12 any more.
>>> 
>>> But I think I can create a situation with 6.5 only that reproduces the 
>>> error. Suppose I have a brick that 99% full.  So a write() will succeed. 
>>> After the write, the brick can be 100% full, so a subsequent mkdir() will 
>>> produce stale file handles (i.e., bricks that have different directory 
>>> trees).  The funny thing is, that the mkdir() on the user side does not 
>>> produce an error.   Clearly, no-one should ever let the file system get to 
>>> 99%, but still, mkdir should fail then... 
>> 
>> I think there is a soft and hard limit that prevents creation of 
>> files/folders when a specific threshold is hit , but that threshold might be 
>> per brick instead of per replica set.
> 
> There is the cluster.min-free-disk, which states that the server should look 
> for a free brick if the hash would place the file on a brick with less than 
> "min-free-disk" bytes.   However, this seems to be a "should". If all bricks 
> have less space than "min-free-disk", then the file is written anyway. 
> 
> Apart from that, I have some really large bricks (around 200 TB each), which 
> means that if these are 99% full, then there are still 2 TB left (a 
> signifikant amount).  The logic of "do not create a directory if the brick is 
> 100% full" seems to be hard coded.  I didn't find a setting to disable this 
> logic.

I actually found a setting that influences this behavior:
setting

gluster v set <volname> storage.reserve  0

lets me fill the bricks to 100%   So my issue is solved.
best wishes,
-Stefan


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