[Ocfs2-devel] what is global bitmap, block group, suballocator, etc

[Tao Ma]

Coly Li wrote:
> Hi Tao,
> 
> Thanks for your reply. I spent some time to read the code and understand :)
> 
> On 2009年11月25日 15:59, Tao Ma Wrote:
>> Hi Coly,
>>
>> Coly Li wrote:
>>> Hi list,
>>>
>>> There are some ocfs2 terminologies that I don't understand very well.
>>> After checking the ocfs2 wiki and kernel/tools
>>> code, I am still not sure whether I understand them correctly.
>>>
>>> - Global bitmap
>>>   What's the usage of global bitmap? Which kind of resource allocation
>>> will set global bitmap?
>> global bitmap is a system file which indicates the usage of clusters in
>>  the whole volume. It is a chain file. So it has a ocfs2_chain_list in
>> its ocfs2_dinode.id2. And the chain_list is a chain of many groups. A
>> group is a range of clusters and the 1st block of a group has a bitmap
>> to indicate which cluster(or block, cpg is used here) is allocated or not.
> 
> What is a chain file ? Is it a file not for regular file data storage (a.k.a dinode->i_size == 0), but to record
> allocation chains of an ocfs2 volume (a.k.a dinode->id2.i_chain) ?
yeah, it is used in system file only. So when you allocate some clusters 
from the global_bitmap, you add it to your own chain file according to 
the usage.
> 
> Another question is, for each node (slot), should it have only 1 chain file ? If yes, I guess the chain number limit is
> ocfs2_chain_recs_per_inode().
No, you have different system files for different usage.
Please check the flag of the system file, it should have OCFS2_CHAIN_FL 
set. You can also investigate it by debugfs.ocfs2.
e.g
echo 'stat //inode_alloc:0000'|debugfs.ocfs2 /dev/sdx
you can see "Chain" in the "Flags:" field.

So normally we have inode_alloc:000X for inode allocation, 
extent_alloc:000X for metadata allocation,

Regards,
Tao