> So I see several problems with this. Firstly, it breaks the ENOSPC handling > logic which relies on the fact that after forcing a transaction commit all > blocks held by the transaction are released - now they will be released > only after the work is completed and thus we can prematurely report ENOSPC. Hi, Jan. We already freed all the blocks in the block bitmap and increased sbi->s_freeclusters_counter in ext4_free_blocks() in advance of ext4_free_data_callback() which is handling discard commands and releasing blocks in the buddy cache. So, I think that it's ok about ENOSPC, because we are ckecking whether we can allocate the blocks or not using ext4_claim_free_clusters(), which is just seeing sbi->s_freeclusters_counter, and the blocks were already freed from on-disk block bitmap. > Secondly, offloading the discard work doesn't change the fundamental fact > that discard is slow (for some devices) and this change just hides this > fact at the possible cost of for example higher file fragmentation as a > result of delayed block freeing. Also the outstanding queue of discard > requests isn't limited in any way again leading to possible strange > allocation / ENOSPC issues. Yes, I agree with you about that the discard handling will be still slow. However, by hiding this, we can get a better responsiveness of fsync() from 30s to 0.255s in the worst case and this is very important to mobile environments where fsync() delay means the users have to wait to do the next action in a while. For the higher file fragmentation, even now, we cannot free the blocks fastly in the buddy cache because we have to handle all the discard commands before freeing blocks in the buddy. So, we already have the same problem now. :-) Thank you.
