On Fri, 8 Jul 2011, Kyungmin Park wrote: > On Thu, Jul 7, 2011 at 11:41 PM, Lukas Czerner <lczerner@xxxxxxxxxx> wrote: > > 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. > Can you let me know the link? I am sure google can find it for you without any problems. Regards! -Lukas
