Re: [f2fs-dev] Can F2FS roll forward after fdatasync()?

[Jaegeuk Kim <jaegeuk@kernel.org>]

Hi Hongwei,

On 05/29, Hongwei wrote:
> Hi,
> >On 05/28, Hongwei wrote:
> >> Hi F2FS experts,
> >> As written in f2fs_do_sync_file():
> >> "Both of fdatasync() and fsync() are able to be recovered from sudden-power-off."
> >>
> >> Please consider this workflow:
> >> 1. Start atomic write
> >> 2. Multiple file writes
> >> 3. Commit atomic write
> >> 4. fdatasync()
> >> 5. Powerloss.
> >>
> >> In the 4th step, the fdatasync() doesn't wait for node writeback.
> >> So we may loss node blocks after powerloss.
> >>
> >> If the data blocks are persisted but node blocks aren't, can the recovery program recover the transaction?
> >
> >#3 will guarantee the blocks written by #2. So, if there's no written between #3
> >and #4, I think we have nothing to recover.
> >Does this make sense to you?
> 
> Thanks for your reply. Please consider this:
> f2fs_do_sync_file() doesn't wait for node writeback if atomic==1. So it is possible that after #3, node is still writing back.
> #4 fdatasync() doesn't wait for node write back either.
> Considering node writeback BIO is flagged with PREFLUSH and FUA, it may take a long time to complete.
> Therefore, when #5 power failure happens, it is possible that the node block is not persisted?
> If I was correct about this, can the recovery program recover the transaction?

I see. That can be the issue tho, is there a real usecase for this? I mean,
given atomic writes by sqlite, next transaction will be also serialized with
another atomic writes, which we could bypass waiting node writes.

Thanks,

> 
> >
> >>
> >> Thanks!
> >>
> >> Hongwei
> >> _______________________________________________
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> >> Linux-f2fs-devel@lists.sourceforge.net
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