On 02/20/2014 08:38 PM, Wang Shilong wrote:
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.
This should be related to btrfs free space cache, it is designed as
nocow without
checksums.
Thanks,
Wang
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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