delalloc and reservation.

["Aneesh Kumar K.V" <aneesh.kumar@linux.vnet.ibm.com>]

Hi All,

I looked at the delalloc and reservation differences that Valerie was observing.
Below is my understanding. I am not sure whether the below will result in 
higher fragmentation that Eric Sandeen is observing. I guess it should not. Even
though the reservation gets discarded during the clear inode due to memory pressure
the request for new reservation should get the blocks nearby and not break extents right ?


any how below is the simple case.

without delalloc the blocks are requested during prepare_write/write_begin.
That means we enter ext4_new_blocks_old which will call ext4_try_to_allocate_with_rsv.
Now if there is no reservation for this inode a new one will be allocated.  After
using the blocks this reservation is destroyed during the close via ext4_release_file

With delalloc the blocks are not requested until we hit writeback/ext4_da_writepages
That means if we create new file and close them the reservation will be discarded
during close via ext4_release_file.( Actually there will be nothing to clear)
Now when we do a sync/or write back. We try to get the block, the inode will
request for new reservation. This reservation is not discarded untill we call clear_inode
and that results in the behavior we are seeing. 

Free blocks: 1440-8191, 8194-8199, 8202-8207, 8210-8215, 8218-8223, 8226-8231, 8234-8239, 8242-8247, 8250-8255, 8258-8263, 8266-8271, 8274-8279, 8282-8287, 8290-8295, 8298-8303, 8306-8311, 8314-8319, 8322-8327, 8330-8335, 8338-8343, 8346-12799

So now the question is where do we discard the reservation in case of delalloc.

-aneesh