Re: [PATCH 1/2] f2fs: use crc and cp version to determine roll-forward recovery

From: Jaegeuk Kim
Date: Thu Sep 29 2016 - 20:53:16 EST

On Thu, Sep 29, 2016 at 08:01:32PM +0800, Chao Yu wrote:
> On 2016/9/20 10:55, Jaegeuk Kim wrote:
> > Previously, we used cp_version only to detect recoverable dnodes.
> > In order to avoid same garbage cp_version, we needed to truncate the next
> > dnode during checkpoint, resulting in additional discard or data write.
> > If we can distinguish this by using crc in addition to cp_version, we can
> > remove this overhead.
> >
> > There is backward compatibility concern where it changes node_footer layout.
> > But, it only affects the direct nodes written after the last checkpoint.
> > We simply expect that user would change kernel versions back and forth after
> > stable checkpoint.
> Seems with new released v4.8 f2fs, old image with recoverable data could be
> mounted successfully, but meanwhile all fsynced data which needs to be recovered
> will be lost w/o any hints?
> Could we release a new version mkfs paired with new kernel module, so we can tag
> image as a new layout one, then new kernel module can recognize the image layout
> and adjust version suited comparing method with old or new image?

Hmm, how about adding a checkpoint flag like CP_CRC_RECOVERY_FLAG?
Then, we can proceed crc|cp_ver, if the last checkpoint has this flag.

Any thought?

> Thanks,