Re: [Ext2-devel] [RFC] Adding multiple block allocation

[Mingming Cao <cmm@us.ibm.com>]

On Fri, 2005-04-29 at 10:10 -0700, Mingming Cao wrote:
> But if it try to allocating blocks in the hole (with direct IO), blocks
> are allocated one by one. I am looking at it right now.
> 
Hi Andrew, Badari,

If we do direct write(block allocation) to a hole, I found that the
"create" flag passed to ext3_direct_io_get_blocks() is 0 if we are
trying to _write_ to a file hole. Is this expected? 

This is what happened on mainline 2.6.12-rc2(and with my patch). To simplify, here is the problem description on mainline:

If we do 30 blocks write to a new file at offset 800k, fine, create flag is all 1. 
Then if seek back to offset 400k, write another 30 blocks, create flag is 0

-bash-2.05b# mount -t ext3 /dev/ubdc /mnt/ext3
-bash-2.05b# cd /mnt/ext3
-bash-2.05b# touch a
-bash-2.05b# /root/filetst -o 819200 -b 122880 -c 1 -w -d -f a
Calling ext3_get_block_handle from ext3_direct_io_get_blocks: maxblocks = 30, iblock = 200, create = 1
Calling ext3_get_block_handle from ext3_direct_io_get_blocks: maxblocks = 29, iblock = 201, create = 1
Calling ext3_get_block_handle from ext3_direct_io_get_blocks: maxblocks = 28, iblock = 202, create = 1
Calling ext3_get_block_handle from ext3_direct_io_get_blocks: maxblocks = 27, iblock = 203, create = 1
Calling ext3_get_block_handle from ext3_direct_io_get_blocks: maxblocks = 26, iblock = 204, create = 1
...................
Calling ext3_get_block_handle from ext3_direct_io_get_blocks: maxblocks = 5, iblock = 225, create = 1
Calling ext3_get_block_handle from ext3_direct_io_get_blocks: maxblocks = 4, iblock = 226, create = 1
Calling ext3_get_block_handle from ext3_direct_io_get_blocks: maxblocks = 3, iblock = 227, create = 1
Calling ext3_get_block_handle from ext3_direct_io_get_blocks: maxblocks = 2, iblock = 228, create = 1
Calling ext3_get_block_handle from ext3_direct_io_get_blocks: maxblocks = 1, iblock = 229, create = 1

-bash-2.05b# /root/filetst -o 409600 -b 122880 -c 1 -w -d -f a
Calling ext3_get_block_handle from ext3_direct_io_get_blocks: maxblocks = 30, iblock = 100, create = 0


Because of create flag is 0, ext3_get_block will not do block allocation
and return immediately after look up failed. Then ext3_get_block_handle
() is called from other path(I am not sure where) other than
ext3_direct_io_get_blocks to allocate the desired 30 blocks.(thus, when
apply ext3_get_blocks patch, ext3_get_blocks is not called)

Could you clarify?

Thanks,
Mingming

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