On Wed, Mar 30, 2011 at 12:33 PM, Joel Becker [off-list ref] wrote:

On Wed, Mar 30, 2011 at 08:05:38AM +0200, Amir Goldstein wrote:

quoted

Just wanted to clarify there are 2 differences I notice between mmap
write to a hole
and mmap write to COWed file with ENOSPC:

1. A "good" application can avoid mmap write to a hole.

2. when initiating a hole, the mkwrite callback is in used (in ext4) to
reserve disk space for delayed allocation when a page becomes writable.
with COW a page may already be writable when the flush encounters COW
with ENOSPC. that flush can even happen after the application has exited,
so the data will be dropped on the floor silently (like in ext3).

       ocfs2 doesn't have delayed allocation yet, so we try and fail
the allocation in write_begin() right under mkwrite().

And what if the page is already writable?
Do you go over all inode pages and make them RO after fastcopy?
For volume level snapshot this isn't a sensible option.

Amir.
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