On Tue, 27 Jun 2017, Ning Yao wrote:
> Hi, all
>
> currently I find that when do copy on write for a clone image. librbd
> call the cls copyup function to write the data, reading from its
> parent, to the child.
>
> However, there is a issue here: if an object in the parent image -->
> [0, 8192] with data and [8192, end] without data, then after COW
> operation, it will filling the whole object [0, end] to the children
> object with [8192, end] all zeros. This phenomenon also occurs in
> flatten images.
Note that BlueStore (luminous) doesn't have this issue: the clone is an
O(1) metadata operation and subsequent writes are basically copy-no-write.
> Actually, we already have sparse_read to just read data without holes.
> However, copyup function does not support to write serveral fragments
> such as {[0, 8192], [16384,20480]}.
>
> So it that possible to direct send OSDOp {[cow write], [cow write],
> [user write]} instead of OSDOp {[copyup], [user write]} ?
It seems like the better fix for FileStore is to make the copyup operation
do a sparse_read and write only the allocated ranges. I think the only
issue there is that the two mechanisms for making sparse_read actually
sparse are fiemap and seek_hole_data, both of which are disabled by
default because they rely on newish or buggy-in-the-past kernel APIs and
we want to avoid hard to diagnose breakage. They should be enabled with
caution.
sage
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