Guys, Mike and Sreenivasa at google are looking into implementing
fallocate() on ext2. Of course, any such implementation could and should
also be portable to ext3 and ext4 bitmapped files.
I believe that Sreenivasa will mainly be doing the implementation work.
The basic plan is as follows:
- Create (with tune2fs and mke2fs) a hidden file using one of the
reserved inode numbers. That file will be sized to have one bit for each
block in the partition. Let's call this the "unwritten block file".
The unwritten block file will be initialised with all-zeroes
- at fallocate()-time, allocate the blocks to the user's file (in some
yet-to-be-determined fashion) and, for each one which is uninitialised,
set its bit in the unwritten block file. The set bit means "this block
is uninitialised and needs to be zeroed out on read".
- truncate() would need to clear out set-bits in the unwritten blocks file.
- When the fs comes to read a block from disk, it will need to consult
the unwritten blocks file to see if that block should be zeroed by the
CPU.
- When the unwritten-block is written to, its bit in the unwritten blocks
file gets zeroed.
- An obvious efficiency concern: if a user file has no unwritten blocks
in it, we don't need to consult the unwritten blocks file.
Need to work out how to do this. An obvious solution would be to have
a number-of-unwritten-blocks counter in the inode. But do we have space
for that?
(I expect google and others would prefer that the on-disk format be
compatible with legacy ext2!)
- One concern is the following scenario:
- Mount fs with "new" kernel, fallocate() some blocks to a file.
- Now, mount the fs under "old" kernel (which doesn't understand the
unwritten blocks file).
- This kernel will be able to read uninitialised data from that
fallocated-to file, which is a security concern.
- Now, the "old" kernel writes some data to a fallocated block. But
this kernel doesn't know that it needs to clear that block's flag in
the unwritten blocks file!
- Now mount that fs under the "new" kernel and try to read that file.
The flag for the block is set, so this kernel will still zero out the
data on a read, thus corrupting the user's data
So how to fix this? Perhaps with a per-inode flag indicating "this
inode has unwritten blocks". But to fix this problem, we'd require that
the "old" kernel clear out that flag.
Can anyone propose a solution to this?
Ah, I can! Use the compatibility flags in such a way as to prevent the
"old" kernel from mounting this filesystem at all. To mount this fs
under an "old" kernel the user will need to run some tool which will
- read the unwritten blocks file
- for each set-bit in the unwritten blocks file, zero out the
corresponding block
- zero out the unwritten blocks file
- rewrite the superblock to indicate that this fs may now be mounted
by an "old" kernel.
Sound sane?
- I'm assuming that there are more reserved inodes available, and that
the changes to tune2fs and mke2fs will be basically a copy-n-paste job
from the `tune2fs -j' code. Correct?
- I haven't thought about what fsck changes would be needed.
Presumably quite a few. For example, fsck should check that set-bits
in the unwriten blobks file do not correspond to freed blocks. If they
do, that should be fixed up.
And fsck can check each inodes number-of-unwritten-blocks counters
against the unwritten blocks file (if we implement the per-inode
number-of-unwritten-blocks counter)
What else should fsck do?
- I haven't thought about the implications of porting this into ext3/4.
Probably the commit to the unwritten blocks file will need to be atomic
with the commit to the user's file's metadata, so the unwritten-blocks
file will effectively need to be in journalled-data mode.
Or, more likely, we access the unwritten blocks file via the blockdev
pagecache (ie: use bmap, like the journal file) and then we're just
talking direct to the disk's blocks and it becomes just more fs metadata.
- I guess resize2fs will need to be taught about the unwritten blocks
file: to shrink and grow it appropriately.
That's all I can think of for now - I probably missed something.
Suggestions and thought are sought, please.
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