Re: [PATCH 2/2] Add batched discard support for ext4.

[Jeff Moyer <jmoyer@redhat.com>]

Lukas Czerner <lczerner@redhat.com> writes:

> On Mon, 26 Apr 2010, Ric Wheeler wrote:
>
>> On 04/26/2010 01:46 PM, Lukas Czerner wrote:
>> > On Mon, 26 Apr 2010, Jan Kara wrote:
>> > 
>> > > > On Wed, 21 Apr 2010, Greg Freemyer wrote:
>> > > > And also, currently I am rewriting the patch do use rbtree instead of
>> > > > the
>> > > > bitmap, because there were some concerns of memory consumption. It is a
>> > > > question whether or not the rbtree will be more memory friendly.
>> > > > Generally I think that in most "normal" cases it will, but there are
>> > > > some
>> > > > extreme scenarios, where the rbtree will be much worse. Any comment on
>> > > > this ?
>> > >    I see two possible improvements here:
>> > > a) At a cost of some code complexity, you can bound the worst case by
>> > > combining
>> > > RB-trees with bitmaps. The basic idea is that when space to TRIM gets too
>> > > fragmented (memory to keep to-TRIM blocks in RB-tree for a given group
>> > > exceeds
>> > > the memory needed to keep it in a bitmap), you convert RB-tree for a
>> > > problematic group to a bitmap and attach it to an appropriate RB-node. If
>> > > you
>> > > track with a bitmap also a number of to-TRIM extents in the bitmap, you
>> > > can
>> > > also decide whether it's benefitial to switch back to an RB-tree.
>> > 
>> > This sounds like a good idea, but I wonder if it is worth it :
>> >   1. The tree will have very short life, because with next ioctl all
>> >   stored deleted extents will be trimmed and removed from the tree.
>> >   2. Also note, that the longer it lives the less fragmented it possibly
>> >   became.
>> >   3. I do not expect, that deleted ranges can be too fragmented, and
>> >   even if it is, it will be probably merged into one big extent very
>> >   soon.
>> > 
>> > > 
>> > > b) Another idea might be: When to-TRIM space is fragmented (again, let's
>> > > say
>> > > in some block group), there's not much point in sending tiny trim commands
>> > > anyway (at least that's what I've understood from this discussion). So you
>> > > might as well stop maintaining information which blocks we need to trim
>> > > for that group. When the situation gets better, you can always walk block
>> > > bitmap and issue trim commands. You might even trigger this rescan from
>> > > kernel - if you'd maintain number of free block extents for each block
>> > > group
>> > > (which is rather easy), you could trigger the bitmap rescan and trim as
>> > > soon
>> > > as ratio number of free blocks / number of extents gets above a reasonable
>> > > threshold.
>> > > 
>> > > 								Honza
>> > > 
>> > 
>> > In what I am preparing now, I simple ignore small extents, which would
>> > be created by splitting the deleted extent into smaller pieces by chunks
>> > of used blocks. This, in my opinion, will prevent the fragmentation,
>> > which otherwise may occur in the longer term (between ioctl calls).
>> > 
>> > Thanks for suggestions.
>> > -Lukas
>> 
>> I am not convinced that ignoring small extents is a good idea. Remember that
>> for SSD's specifically, they remap *everything* internally so our
>> "fragmentation" set of small spaces could be useful for them.
>> 
>> That does not mean that we should not try to send larger requests down to the
>> target device which is always a good idea I think :-)
>> 
>> ric
>> 
>
> That's right, so the other approach would be probably better. Merge
> small extents together into one, but there must be some limit, because I
> do not want two little extents at the beginning and the end of the group
> to force trimming whole group. The whole rbtree thing gets a little
> complicated :)

This discussion is getting a bit too abstract for me.  Show us the code
and we can make some progress.  =)

On the topic of discarding small blocks, I agree with Ric, it should be
done.

Cheers,
Jeff