On 02/20/2014 07:25 PM, Sebastian Ochmann wrote:
Hello,
Sebastian Ochmann posted on Wed, 19 Feb 2014 13:58:17 +0100 as
excerpted:
So my question is, why does scrub show a high (i.e. non-zero)
value for
no_csum? I never enabled nodatasum or a similar option.
Did you enable nodatacow option? if nodatacow option is enabled,
data checksums will be also disabled at the same time.
[SNIP]
So i have found why we have such strange things from debugging, you can
have a try as the
following steps:
# mkfs.btrfs -f /dev/sda9
# mount /dev/sda9 /mnt
# btrfs scrub start -BR /mnt
scrub done for 02dd3326-959f-4602-9baa-aa9ed99ac2e5
scrub started at Thu Feb 20 20:31:46 2014 and finished after 0 seconds
data_extents_scrubbed: 1
tree_extents_scrubbed: 16
data_bytes_scrubbed: 65536
tree_bytes_scrubbed: 262144
read_errors: 0
csum_errors: 0
verify_errors: 0
no_csum: 16 ---------------------->not equal 0 for a newly mkfs.
csum_discards: 0
super_errors: 0
malloc_errors: 0
uncorrectable_errors: 0
unverified_errors: 0
corrected_errors: 0
last_physical: 467140608
# btrfs-debug-tree /dev/sda9
By debuging tree, we found there is a EXTENT_ITEM in extent tree for newly
mkfs filesystem which we have written anything yet.
item 2 key (12582912 EXTENT_ITEM 65536) itemoff 16182 itemsize 53
extent refs 1 gen 5 flags 1
extent data backref root 1 objectid 256 offset 0 count 1
item 3 key (12582912 BLOCK_GROUP_ITEM 8388608) itemoff 16158 itemsize 24
At the same time, Let's see Csum tree debug output:
checksum tree key (CSUM_TREE ROOT_ITEM 0)
leaf 29425664 items 0 free space 16283 generation 4 owner 7
fs uuid 02dd3326-959f-4602-9baa-aa9ed99ac2e5
So there is not corresponding checksum item for that DATA extent item..
This can explain why scrub output no_sum count!
For reasons, It may be reserved data space without checksum for other
purpose!
So if this is true, i don't think this is harm for common users unless
it is designed
unexpectedly!
Thanks,
Wang
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