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 >
