Btrfs devel - Jun 2007 - checksum errors

Hello,

I'm getting errors when I mount a freshly mkfs'ed btrfs filesystem

btrfs: dm-2 checksum verify failed on 4

and so on for however many roots were created.  It looks to be because mkfs
doesn't create checksums for the roots when its making the filesystem, so
all of
them will have null checksum feilds at first, until eventually they get
populated by somebody writing or something like that.  Should mkfs be creating
these checksums when its first creating the filesystem?  Just want to know so I
know where I should be trying to fix this, or if I should mess with it at all.
Thank you,

Josef

On Wed, Jun 20, 2007 at 03:07:10PM -0400, Josef Bacik
wrote:
> Hello,
> 
> I'm getting errors when I mount a freshly mkfs'ed btrfs filesystem
> 
> btrfs: dm-2 checksum verify failed on 4
> 
> and so on for however many roots were created.  It looks to be because mkfs
> doesn't create checksums for the roots when its making the filesystem,
so all of
> them will have null checksum feilds at first, until eventually they get
> populated by somebody writing or something like that.
Exactly, I haven't put the csum code into mkfs yet.
> Should mkfs be creating
> these checksums when its first creating the filesystem?  Just want to know
so I
> know where I should be trying to fix this, or if I should mess with it at
all.
> Thank you,
Yes, we definitely need this in userland, it should be a pretty quick
mini-project.  The easy way is to change disk-io.c:write_tree_block to
do the csum just before it calls pwrite.  See csum_tree_block and
btrfs_csum_data in the kernel module for a basic idea.

For all the tree blocks, the csum field is the first 32 bytes (even if
you're only doing a 4 byte crc32c csum).  So, the csum field is not fed
into the csum function, but the entire rest of the block is.

Longer term, there needs to be a field in the tree root that indicates
what kind of csum is used for all the blocks in the tree.  For now, you
can just assume crc32c.

-chris

I'm interested in implementing the checksum calculation in mkfs,
however, I have problems using the crc32c function. In what library is
it defined?

Thanks,

Frank Groeneveld