On Wed, Sep 21, 2016 at 8:08 PM, Qu Wenruo [off-list ref] wrote:

At 09/21/2016 11:13 PM, Chris Murphy wrote:

quoted

I understand some things should go in fsck for comparison. But in this
case I don't see how it can help. Parity is not checksummed. The only
way to know if it's wrong is to read all of the data strips, compute
parity, and compare in-memory parity from current read to on-disk
parity.

That's what we plan to do.
And I don't see the necessary to csum the parity.
Why csum a csum again?

parity!=csum


http://www.spinics.net/lists/linux-btrfs/msg56602.html

quoted

There is already an offline scrub in btrfs
check which doesn't repair, but also I don't know if it checks parity.

       --check-data-csum
           verify checksums of data blocks

Just as you expected, it doesn't check parity.
Even for RAID1/DUP, it won't check the backup if it succeeded reading the
first stripe.

Both copies are not scrubbed? Oh hell...

[chris@f24s ~]$ sudo btrfs scrub status /brick2
scrub status for 7fea4120-9581-43cb-ab07-6631757c0b55
    scrub started at Tue Sep 20 12:16:18 2016 and finished after 01:46:58
    total bytes scrubbed: 955.93GiB with 0 errors

How can this possibly correctly say 956GiB scrubbed if it has not
checked both copies? That message is saying *all* the data, both
copies, were scrubbed. You're saying that message is wrong? It only
scrubbed half that amount?

[chris@f24s ~]$ sudo btrfs fi df /brick2
Data, RAID1: total=478.00GiB, used=477.11GiB
System, RAID1: total=32.00MiB, used=96.00KiB
Metadata, RAID1: total=2.00GiB, used=877.59MiB
GlobalReserve, single: total=304.00MiB, used=0.00B


When that scrub was happening, both drives were being accessed at 100%
throughput.

Current implement doesn't really care if it's the data or the copy
corrupted, any data can be read out, then there is no problem.
The same thing applies to tree blocks.

So the ability to check every stripe/copy is still quite needed for that
option.

And that's what I'm planning to enhance, make --check-data-csum to kernel
scrub equivalent.

OK thanks.

quoted

           This expects that the filesystem is otherwise OK, so this
is basically and
           offline scrub but does not repair data from spare coipes.

Repair can be implemented, but maybe just rewrite the same data into the
same place.
If that's a bad block, then it can't repair further more unless we can
relocate extent to other place.

Any device that's out of reserve sectors and can no longer remap LBA's
on its own, is a drive that needs to be decommissioned. It's a new
feature in just the last year or so that mdadm has a badblocks map so
it can do what the drive won't do, but I'm personally not a fan of
keeping malfunctioning drives in RAID.

quoted

Is it possible to put parities into their own tree? They'd be
checksummed there.

Personally speaking, this is quite a bad idea to me.
I prefer to separate different logical layers into their own codes.
Not mixing them together.

Block level things to block level(RAID/Chunk), logical thing to logical
level(tree blocks).

OK.

Current btrfs csum design is already much much better than pure RAID.
Just think of RAID1, while one copy is corrupted, then which copy is correct
then?

Yes.


-- 
Chris Murphy