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On 8/26/15 12:04 AM, Jun He wrote:
> Hi, I have been playing with btrfs discard for a while and found
> that btrfs may fail to discard some extents with 'mount -o
> discard'. I am aware of Jeff Mahoney's patches (
> https://patchwork.kernel.org/patch/6609491/ ). It seems that the
> patches do not fix the problem. I have seen the same problematic 
> behavior for the following versions
> 
> - https://git.kernel.org/cgit/linux/kernel/git/fdmanana/linux.git/ 
> integration-4.3 commit:477594f93c43b1ee685 - 3.16.0 - 4.2.0-rc7
> 
> The problem can be reproduced by writing and fsyncing a 4MB file
> for 50 times on a 256MB empty FS (mount option: -o discard). You
> will find that some extents are not discarded (my expected behavior
> is that, after overwriting, an old version of a file extent should
> be discarded). I use several ways to confirm this:
> 
> 1. I created a loop device back by a sparse file in tmpfs. After
> running the workload, I found the file is 29MB (ls -lsh). If you
> fstrim the file system, the sparse file will become 4.1MB. This
> proves that there are a lot of data not discarded.

Can you capture the output of 'filefrag -b -v' for the sparse file in
tmpfs both before and after trim?  Also btrfs-debug-tree output for it.

> 2. I collected blktrace + blkparse output and plotted the write and
> discard operations in a space-time graph, where you can intuitively
> see some extents are overwritten but not discarded. Here is the
> space-time graph 
> https://gist.githubusercontent.com/junhe/b6ce39eeb6de8887e66a/raw/825a
3c2946b52a50c2b6032a98d637f5a32bc5c3/integration-4.3.png
>
>  Is it a known problem or is it not a problem? If it is a known
> problem and there exists a patch that I am not aware of, can
> somebody direct me to it? If it is specifically designed this way,
> can the designers give the rationale of discarding some, but not
> all of, old extents?
> 
> The reproducer can be run by: git clone
> https://gist.github.com/b6ce39eeb6de8887e66a.git cd
> b6ce39eeb6de8887e66a sudo python main.py
> 
> It also has a readme.

I tested my discard patches the other day against 4.2-rc8 and it
passed my 326 test.  I'll check out your reproducer.

- -Jeff



- -- 
Jeff Mahoney
SUSE Labs
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