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On 5/1/15 7:55 AM, Uwe Sauter wrote:
>
>
> Am 01.05.2015 um 02:21 schrieb Samuel Merritt:
>>
>> It seems like 1430268763.41931.data would be in the same allocation group as
>> objects/757/a94/bd77129a1cae9e32381776e322efca94, and bd77129a1cae9e32381776e322efca94 would be in the same allocation
>> group as objects/757/a94, and so on. Thus, everything would be in the same allocation group as the root directory.
>>
>> This can't be the case, or else there'd be no point to allocation groups. What am I missing here?
>
>
> Hi,
>
> I think what you're missing is, that inodes stay in the allocation group where they first were created. So moving a file
> around in the filesystem changes the path but not the allocation group. So first creating a temporary file and then
> moving it into the hash folder leaves the file associated with the temp folder's allocation group, thus the allocation
> group grows bigger and bigger and searching the allocation group takes more and more time.

That doesn't really answer the question, though. We have this message 
<http://www.spinics.net/lists/xfs/msg32868.html> which says that "...the 
locality of a new inode is determined by the
parent inode, and so if all new inodes are created in the same
directory, then they are all created in the same AG."

Let's say we start out with a freshly-formatted disk, so there's only 
one inode, and it's for the root directory.

Then, Swift goes and starts making its directory structure on disk, and 
calls mkdir('objects'). Since a new inode is created in the same AG as 
its parent, the inode for '/objects' is in the same AG as the inode for '/'.

Swift makes another dir: mkdir('objects/757')

The inode for '/objects/757' is in the same AG as its parent '/objects', 
which is the same as the AG for '/'.

Keep going a while, and you get

/
/objects
/objects/757
/objects/757/a94
/objects/757/a94/bd77129a1cae9e32381776e322efca94
/objects/757/a94/bd77129a1cae9e32381776e322efca94/1430268763.41931.data
/tmp

and they're all in the same AG.

Now, the XFS developers are not stupid, so what I typed up there can't 
possibly be true, or else every inode on a filesystem would be in the 
same AG.

So, my question is this: what, of the things I typed above, is false? 
Equivalently, how is an inode created in a *different* AG than its parent?