Re: [f2fs-dev] Potential data corruption?

[Chao Yu <chao@kernel.org>]

Hello,

On 2019-12-7 18:10, 红烧的威化饼 wrote:
> Hi F2FS experts,
> The following confuses me:
>
> A typical fsync() goes like this:
> 1) Issue data block IOs
> 2) Wait for completion
> 3) Issue chained node block IOs
> 4) Wait for completion
> 5) Issue flush command
>
> In order to preserve data consistency under sudden power failure, it requires that the storage device persists data blocks prior to node blocks.
> Otherwise, under sudden power failure, it's possible that the persisted node block points to NULL data blocks.

Firstly it doesn't break POSIX semantics, right? since fsync() didn't return
successfully before sudden power-cut, so we can not guarantee that data is fully
persisted in such condition.

However, what you want looks like atomic write semantics, which mostly database
want to guarantee during db file update.

F2FS has support atomic_write via ioctl, which is used by SQLite officially, I
guess you can check its implementation detail.

Thanks,

>
> However, according to this study (https://www.usenix.org/conference/fast18/presentation/won), the persistent order of requests doesn't necessarily equals to the request finish order (due to device volatile caches). This means that its possible that the node blocks get persisted prior to data blocks.
>
> Does F2FS have other mechanisms to prevent such inconsistency? Or does it require the device to persist data without reordering?
>
> Thanks!
>
> Hongwei
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>


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