Re: inquiry on btrfs send/receive

[Arne Jansen <sensille@gmx.net>]

On 04.06.2012 14:39, Alex Lyakas wrote:

> 
> # How does one track changes in generic INODE_ITEM properties, like
> "mode" or "uid/gid"? Whenever such property gets changed, INODE_ITEM
> gets stamped with a new transid, but do we need to compare it with the
> previous version on the receive side to realize what has changed?
> # File size - is it required, again, to compare vs previous size, to
> realize file truncation? (file grow perhaps can be realized via new
> EXTENT_DATAs)

The basic idea of send/receive is not to find anything that has changed
since a given transid number, but to find the differences between 2
snapshots. This way you always have access to the old values.

> # What should be done if INODE_ITEM::flags change (e.g., inode gets
> nodatacow/nodatasum flags set). What should be done at receive side?

> # How does one track deletion of INODE_ITEMs? Or, deletion and
> re-creation of a INODE_ITEM with the same inode number? (I saw that
> inode_cache mount option allows to re-use inode numbers, so I think it
> can happen.) Does this mean that on receive side, it is required to
> compare contents of each directory vs previous version?

A recreated inode gets a new inode generation number. That's needed
for NFS, otherwise NFS could also not detect this case.

> # What should be done with INODE_ITEMs like block/char device, FIFO or a socket?

Everything that can be created on the dest side, like device files,
should be created.

> # XATTR_ITEMs: although they have a transid stamp, again, need to
> track deletion/re-creation of them. Again by comparing?

as long as they end up identical on the destination, delete/recreate
shouldn't matter.

The rest of the question I leave for Jan and Alexander :)

-Arne

> # INODE_REFs: these seem most tough to me, because they don't have
> transid stamps. How such scenario can be handled: an INODE_ITEM had
> two INODE_REFs with names N1 and N2. But now on the send side, both
> those INODE_REFs were deleted and INODE_REFs N3 and N4 were created.
> Does that mean we need to always compare all INODE_REFs for each
> INODE_ITEM, or we perhaps can use DIR_ITEMs/DIR_INDEXs of parent
> INODE_ITEM to detect changes in INODE_REFs?
> 
> All in all, it looks like the approach of navigating the FS tree and
> trying to *understand* specifically which modifications were
> performed, is quite error-prone. And I am sure there are modifications
> I am not aware about.
> 
> I was wondering, what state your work is in? Is it possible to look at
> some code or prototype, to understand what approach have you taken, or
> perhaps an overall description of the approach?
> 
> Jan, I saw that you provided some new code for backref resolving. Can
> you give a hint of how is that related to the send/receive
> functionality?
> 
> Thanks,
> Alex.
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