Re: [PATCH RFC] block: use discard if possible in blkdev_issue_discard()

[Theodore Ts'o <tytso@mit.edu>]

On Fri, Feb 14, 2014 at 05:05:13AM -0800, Christoph Hellwig wrote:
>  (4): add a new flag to blkdev_issue_zeroout to say if deallocating the
>       blocks is okay, and if yes proceed like (1).
> 
> Requiring blocks to be zeroed, but not wanting to remove the
> provisioning seems like a perfectly valid request, especially from
> userspace (e.g. databases)

That seems reasonable.  Should the default be to try to use discard if
it is available, or to keep the current behavior?  I'm leaning towards
defining BLKDEV_ZEROOUT_DISCARD, and changing all of the existing
calls to blkdev_issue_zeroout() to pass in 0 for the flags.  This
implies that if any file system does want to use discard for things
like fallocate(), they have to explicitly request it.

Which brings up a few additional questions, for which I'd appreciate
suggestions/opinons: 

Should we try to have some kind of informal naming scheme for a mount
option so that file systems can explicitly request this behavior,
i.e. "mount -o zeroout_discard"?  Of course, if we do this, very few
people will probably end up using it.

So should we try to set up some standard hueristics so that for
certain class of devices (such as flash) we default to using zeroout
_discard, and for certain other classes of devices (maybe thinp) we
don't?  Perhaps we should make it be tunable based on the number of
blocks that are being discarded, i.e., "mount -o zeroout_discard=128k"
means to use the zeroout_discard flag if we are trying to zeroout more
than 128k?

Also, is it worth creating a new ioctl, BLKZZEROOUT, or perhaps
BLKZEROOUTF, which takes a flags argument, so we can expose this to
userspace?  I'm not sure it is worth it, since at least for e2fsprogs,
I won't want to depend on the user using a new enough kernel to
support the new ioctl, and while it is a bit painful to query the
/sys/block/sdXX/queue/discard_* files, I'm doing it already for other
reasons.

Cheers,

						- Ted