Hello Miklos,
On 05/06/2015 04:49 PM, Miklos Szeredi wrote:
> Hi Michael,
>
> On Wed, May 6, 2015 at 4:17 PM, Michael Kerrisk (man-pages)
> <mtk.manpages@xxxxxxxxx> wrote:
>> I did some editing of the text and added some details to come up with the
>> following. Could you please check it over? I also have one question below.
>> (I have also added some entries under ERRORS, but I've omitted them here.)
>>
>> RENAME_WHITEOUT (since Linux 3.18)
>> This operation makes sense only for overlay/union
>> filesystem implementations.
>>
>> Specifying RENAME_WHITEOUT creates a "whiteout" object
>> at the source of the rename at the same time as per‐
>> forming the rename. The whole operation is atomic, so
>> that if the rename succeeds then the whiteout will
>> also have been created.
>>
>> A "whiteout" is an object that has special meaning in
>> union/overlay filesystem constructs. In these con‐
>> structs, multiple layers exist and only the top one is
>> ever modified. A whiteout on an upper layer will
>> effectively hide a matching file in the lower layer,
>> making it appear as if the file didn't exist.
>>
>> When a file that exists on the lower layer is renamed,
>> the file is first copied up (if not already on the
>> upper layer) and then renamed on the upper, read-write
>> layer. At the same time, the source file needs to be
>> "whiteouted". The whole operation needs to be done
>>
>> ???
>> After "whitedout", I am tempted to add: "(so that the version of
>> the source file in the lower layer is rendered invisible)".
>> Is that a correct formulation, and is it helpful to add it?
>
> Yes, that's correct, and helpful, I think.
Added. Thanks for confirming, and thanks of course for your original text!
Cheers,
Michael
>> atomically.
>>
>> When not part of a union/overlay, the whiteout appears
>> as a character device with a {0,0} device number.
>>
>> RENAME_WHITEOUT requires the same privileges as creat‐
>> ing a device node (i.e., the CAP_MKNOD capability).
>>
>> RENAME_WHITEOUT can't be employed together with
>> RENAME_EXCHANGE.
>>
>> RENAME_WHITEOUT requires support from the underlying
>> filesystem. Among the filesystems that provide that
>> support are shmem (since Linux 3.18), ext4 (since
>> Linux 3.18), and XFS (since Linux 4.1).
>
> Looks good to me.
>
> Thanks,
> Miklos
>
--
Michael Kerrisk
Linux man-pages maintainer; http://www.kernel.org/doc/man-pages/
Linux/UNIX System Programming Training: http://man7.org/training/
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