jim owens wrote:
> FIEMAP_EXTENT_NO_BYPASS
>
> As in "you can't bypass the filesystem" to directly access it.
Can we also commit to this, when FIEMAP_EXTENT_NO_BYPASS is *not* set:
1. The data at fe_physical, and *will not move* so long as nothing
modifies *that particular file*?
2. Both reading *and writing* the file bypassing the filesystem are ok.
The reason for 1 is that some filesystems may move data when _other_
files are modified. Heck, they may do so when other files are simply
read, or at some random whim. Those filesystems would set
FIEMAP_EXTENT_NO_BYPASS (except for files with chattr 't'), because
fe_physical does not correspond to a *stable* location which a program
can subsequently use.
The reason for 2 is that some filesystems checksum the data and/or
replicate it, and won't be readable if you write to it directly.
In both these cases, O_DIRECT may *sometimes* work even though
directly accessing the physical device is unreliable. So
FIEMAP_EXTENT_NO_BYPASS may be treated as "access the file through the
filesystem, use O_DIRECT if you still want direct access, and fall
back to ordinary file access if that doesn't work".
-- Jamie
--
To unsubscribe from this list: send the line "unsubscribe linux-fsdevel" in
the body of a message to majordomo@xxxxxxxxxxxxxxx
More majordomo info at http://vger.kernel.org/majordomo-info.html
