Re: [PATCH] fat: make discard a mount option

[Christoph Hellwig <hch@lst.de>]

On Thu, Nov 19, 2009 at 03:38:04AM +0900, OGAWA Hirofumi wrote:
> Christoph Hellwig <hch@lst.de> writes:
> 
> > Currently shipping discard capable SSDs and arrays have rather sub-optimal
> > implementations of the command and can the use of it can cause massive
> > slowdowns.  Make issueing these commands option as it's already in btrfs
> > and gfs2.
> 
> Thanks. Looks good to me, I'll apply as is.
> 
> BTW, what value is default for those users? And from users of some
> filesystems, it might be better to do in lower layer or VFS?

The situation is the following:

As of 2.6.32-rc7:

 - btrfs needs the discard option to enable trim, it's off by default
 - gfs2 needs the discard option to enable trim, it's off by default
 - ext4 does discard by default, no option to disable
 - fat does discard by default, no option to disable
 - the swap code does discard by default, no option to disable

Eric has a patch for ext4 to implement the btrfs/gfs2 semantics, and
this is the one for fat.  A discussion is ongoing about swap.

I don't think moving block specific options is a good idea to do in the
VFS.  The other option would be to disable it in the block layer, but
that's not alwas intuitive.