Re: [f2fs-dev] [PATCH 04/10] f2fs: support IO alignment for DATA and NODE writes

[Chao Yu]

Hi Jaegeuk,

On 2016/12/31 2:51, Jaegeuk Kim wrote:
> This patch implements IO alignment by filling dummy blocks in DATA and NODE
> write bios. If we can guarantee, for example, 32KB or 64KB for such the IOs,
> we can eliminate underlying dummy page problem which FTL conducts in order to
> close MLC or TLC partial written pages.
> 
> Note that,
>  - it requires "-o mode=lfs".
>  - IO size should be power of 2, not exceed BIO_MAX_PAGES, 256.
>  - read IO is still 4KB.
>  - do checkpoint at fsync, if dummy NODE page was written.

Which scenario we can benefit from? Any numbers?

I doubt that there are some potential side-effect points:
 - write amplification will be more serious than before
 - free space will be more fragmented since dummy blocks is separated in whole
address space
 - there is less chance to merge small(unaligned) IOs in block layer

Thoughts?

Thanks,