RE: [PATCH 10/16] Btrfs: Avoid trustless page-level-repair in dev-replace

[Zhao Lei <zhaolei@cn.fujitsu.com>]

Hi, Stefan Behrens

Thanks for review these patch.

* From: Stefan Behrens [mailto:sbehrens@giantdisaster.de]
> Sent: Monday, January 19, 2015 8:26 PM
> On Mon, 19 Jan 2015 19:20:59 +0800, Zhaolei wrote:
> > From: Zhao Lei <zhaolei@cn.fujitsu.com>
> >
> > Current code of page level repair for dev-replace can only support
> > io-error, we can't use it in checksum-fail case.
> >
> > We can skip this kind of repair in dev-replace just as we in scrub.
> 
> Why? I see dev-replace as a disk copy operation with integrated support
> to pick the best mirror source device. Therefore the strategy was to
> always write to the target device, even in case of uncorrectable
> checksum errors. Maybe the checksum was wrong, not the data itself, and
> you can repair files later by recalculating the checksum. If you write
> nothing at all, some random, old data remains on the target disk,
> instead of potentially fixable data.
> 
> Therefore this is handled differently for the dev-replace and for the
> scrub case. For scrub, data is only overwritten when the source data is
> verified, since the goal is to repair something. Dev-replace's goal is
> to copy something.
> 
I accept your suggestion, copying from another mirror have more
opportunity to recovery than other way:
1: When checksum data is wrong, can be fixed
2: When other mirror have right data on this block
  (checksum fail may be caused by other block ), can be fixed.

Thanks
Zhaolei

> > Signed-off-by: Zhao Lei <zhaolei@cn.fujitsu.com>
> > Signed-off-by: Miao Xie <miaox@cn.fujitsu.com>
> > ---
> >  fs/btrfs/scrub.c | 8 ++++----
> >  1 file changed, 4 insertions(+), 4 deletions(-)
> >
> > diff --git a/fs/btrfs/scrub.c b/fs/btrfs/scrub.c
> > index 9afc6dd..6cf0dc7 100644
> > --- a/fs/btrfs/scrub.c
> > +++ b/fs/btrfs/scrub.c
> > @@ -1099,6 +1099,10 @@ nodatasum_case:
> >  		}
> >  	}
> >
> > +	/* can only fix I/O errors from here on */
> > +	if (sblock_bad->no_io_error_seen)
> > +		goto did_not_correct_error;
> > +
> >  	/*
> >  	 * for dev_replace, pick good pages and write to the target device.
> >  	 */
> > @@ -1181,10 +1185,6 @@ nodatasum_case:
> >  	 * area are unreadable.
> >  	 */
> >
> > -	/* can only fix I/O errors from here on */
> > -	if (sblock_bad->no_io_error_seen)
> > -		goto did_not_correct_error;
> > -
> >  	success = 1;
> >  	for (page_num = 0; page_num < sblock_bad->page_count; page_num++) {
> >  		struct scrub_page *page_bad = sblock_bad->pagev[page_num];
> >