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

From: Chao Yu
Date: Thu Sep 29 2016 - 21:05:25 EST

On 2016/9/30 8:53, Jaegeuk Kim wrote:
> 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?

Ah, that's better. :)


>> Thanks,
> .