Re: ext4: why init the unused block group at batched discard?

[Lukas Czerner]

On Thu, 7 Jul 2011, Kyungmin Park wrote:

> On Thu, Jul 7, 2011 at 6:24 PM, Lukas Czerner <lczerner@xxxxxxxxxx> wrote:
> > On Wed, 6 Jul 2011, Kyungmin Park wrote:
> >
> >> Hi Lukas,
> >>
> >> During code review batched discard support at ext4. I wonder why do
> >> you init the uninitialized block group during batched discard.
> >> As you know uninitialized block group mean that there's no operation
> >> at these blocks.
> >> So no need to trim it at all.
> >
> > What you're describing is another flag, namely EXT4_BG_BLOCK_UNINIT,
> > which tells us that there was no allocation from that block bitmap since
> > the mkfs (as Amir already pointed out). Flag
> > EXT4_GROUP_INFO_NEED_INIT_BIT simply states that there is no buddy
> > initialized for this group.
> >
> > That said the code is perfectly fine, and it should not affect even the
> > e2fsck which uses EXT4_BG_BLOCK_UNINIT to skip not used block groups
> > since we only change it on allocations.
> >
> > It is true that after the commit
> > 78944086663e6c1b03f3d60bf7610128149be5fc ext4: only load buddy bitmap in
> > ext4_trim_fs() when it is needed
> > we do not longer need to initialize the buddy right away, but wait ontil
> > it is really needed. Actually we do not need it at all, because is when
> > we are going to load the buddy the ext4_mb_load_buddy() will check for
> > the EXT4_GROUP_INFO_NEED_INIT_BIT and will initialize the buddy for us.
> >
> > Yongqiang pointed out that we might use EXT4_BG_BLOCK_UNINIT to skip
> > group as well, but I do not think that it is a good idea, since the
> > initial discard at mkfs time might not be done (we just do not know it),
> > so any assumption like this are not right. Moreover there are patches
> > from Tao Ma which adds the code for skipping groups which has not been
> > freed from since the last fitrim call. Search the list for [PATCH 0/4
> > RESEND]  ext4 trim bug fixes and improvement.
> 
> Thank you for all kind explanations.
> 
> Another consideration is that even though batched discard has little
> overhead it's not good idea trim it all unused blocks at one time.
> since disk used blocks doesn't increased in normal case.
> So how about to remember the last allocated block group and trim it
> until this block group?

Yes, this problem is addressed by patches from Tao as mentioned above,
however it is still waiting for merge.

> 
> To reduce the trim time. I also consider the divide the block groups
> as several trim area e.g., 1 GiB and trim it sequentially.

I am not sure what do you mean. In ext4 allocation groups has a lot
smaller sizes (128M for 4k block size) than 1G. Also you can specify
that you do want to discard just a part of the filesystem, but you
probably did notice that in review, right ?

Thanks!
-Lukas

> 
> These are based on the assumption, eMMC has small resource relatively
> SSD and no need to trim it at one time.
> 
> Thank you,
> Kyungmin Park
>