Re: [PATCH 2/2] f2fs: support revoking atomic written pages

[Jaegeuk Kim]

Hi Chao,

On Tue, Dec 29, 2015 at 11:12:36AM +0800, Chao Yu wrote:
> f2fs support atomic write with following semantics:
> 1. open db file
> 2. ioctl start atomic write
> 3. (write db file) * n
> 4. ioctl commit atomic write
> 5. close db file
> 
> With this flow we can avoid file becoming corrupted when abnormal power
> cut, because we hold data of transaction in referenced pages linked in
> inmem_pages list of inode, but without setting them dirty, so these data
> won't be persisted unless we commit them in step 4.
> 
> But we should still hold journal db file in memory by using volatile write,
> because our semantics of 'atomic write support' is not full, in step 4, we
> could be fail to submit all dirty data of transaction, once partial dirty
> data was committed in storage, db file should be corrupted, in this case,
> we should use journal db to recover the original data in db file.

Originally, IOC_ABORT_VOLATILE_WRITE was supposed to handle commit failures,
since database should get its error literally.

So, the only thing that we need to do is keeping journal data for further db
recovery. But, unfortunately, it seems that something is missing in the
current implementation.

So simply how about this?

A possible flow would be:
1. write journal data to volatile space
2. write db data to atomic space
3. in the error case, call ioc_abort_volatile_writes for both journal and db
 - flush/fsync journal data to disk
 - drop atomic data, and will be recovered by database with journal