When is a block free?

[Chris Worley]

On Wed, Oct 1, 2008 at 12:59 PM, Theodore Tso <tytso at mit.edu> wrote:
> On Wed, Oct 01, 2008 at 12:18:21PM -0600, Chris Worley wrote:
>>
>> I was perusing David Woodhouse's 2.6.27-rc2 kernel at
>> git://git.infradead.org/users/drzeus/discard-2.6.git, and noticed he
>> has the discard built-in to where I was talking about for ext2... so I
>> coded our driver to handle discards, and it works very nicely!!!
>
> I'm not sure what you mean by "our driver"?

Our driver for the ioDrive:

http://fusionio.com/Products.aspx

So far, all I've implemented is the "discard" in the read/write
callback; no barrier, no ioctl.

>
>> The journaling issue you raise is not a show-stopper on the block
>> device side: if the block device has to maintain a couple of blocks
>> that are not really in use, it's no big deal (eventually the blocks
>> will be re-written and the universe will be in order again)... for the
>> users, I can understand if the discard is preserved on the block
>> device, while the fs still thinks there's good data in there (we'll
>> give you back all zeros on read).
>
> It's no issue on the block device side at all, but from the user's
> point of view it can be quite disastrous.
<snip>

Maybe that should effect the priority of implementation for ext[34]?

Chris