ext4 extents

[Mike Gibson]

On Oct 29, 2011, at 6:52 PM, "Peter Teoh" <htmldeveloper at gmail.com> wrote:

> On Sat, Oct 29, 2011 at 6:03 AM, Mike Gibson
> <mike.gibson at storagecraft.com> wrote:
>> I'm updating a user space program that finds free blocks on a ext2/3 filesystem so that it works with ext4.  I'm concerned about how extents might affect my program.  My program currently looks only at the block allocation bitmaps.  Are the block allocation bitmaps still updated when an extent is created?
>> 
> 
> problem is what u see as free blocks may not be seen at the kernel
> level as free block.....since u not holding any locks, there may be
> situation when your block allocation may conflict with that of the
> kernel, then u will get disastrous irrecoverable errors - worst case
> is your userspace data get overwritten by kernel.
> 
> i supposed it is a legacy program, but there exists a small
> probability that this problem will occur.

This is only used when the filesystem is offline, so I'm only interested in the on disk structure.

Mike Gibson