copy-on-write files

copy-on-write files

[Steve French]

Joel,
Looking at your documentation for the new "reflink" syscall (to create
copy-on-write files):
http://git.kernel.org/?p=linux/kernel/git/jlbec/ocfs2.git;a=blob;f=Documentation/filesystems/reflink.txt;h=58a6b3879114c0cb591fb736152b30291037edd2;hb=c04db1c934e67337318b177688c0b882297fdd66

Have you looked at whether this is close enough to the existing (e.g.
Windows NTFS) copy-on-write syntax/feature so that we could use it
under Samba (and on the client side from cifs and in the future smb2).
  Windows added copy-on-write about 9 or 10 years ago and uses it
extensively. although I have not researched enough to see what
creating these would look like over the network.


-- 
Thanks,

Steve

Re: copy-on-write files

[Joel Becker]

On Mon, May 11, 2009 at 12:08:17PM -0500, Steve French wrote:
> Joel,
> Looking at your documentation for the new "reflink" syscall (to create
> copy-on-write files):
> http://git.kernel.org/?p=linux/kernel/git/jlbec/ocfs2.git;a=blob;f=Documentation/filesystems/reflink.txt;h=58a6b3879114c0cb591fb736152b30291037edd2;hb=c04db1c934e67337318b177688c0b882297fdd66
> 
> Have you looked at whether this is close enough to the existing (e.g.
> Windows NTFS) copy-on-write syntax/feature so that we could use it
> under Samba (and on the client side from cifs and in the future smb2).
>   Windows added copy-on-write about 9 or 10 years ago and uses it
> extensively. although I have not researched enough to see what
> creating these would look like over the network.

	I didn't know NTFS had it, I don't know the syntax and behavior,
and I wonder where Windows uses it (I haven't encountered it on Windows
machines, but perhaps it is hidden).  Do you have a pointer to any
non-encumbered documentation about the feature?

Joel

-- 

"If the human brain were so simple we could understand it, we would
 be so simple that we could not."
	- W. A. Clouston

Joel Becker
Principal Software Developer
Oracle
E-mail: joel.becker@oracle.com
Phone: (650) 506-8127

Re: copy-on-write files

[Steve French]

On Mon, May 11, 2009 at 12:53 PM, Joel Becker <Joel.Becker@oracle.com> wrote:
> On Mon, May 11, 2009 at 12:08:17PM -0500, Steve French wrote:
>> Joel,
>> Looking at your documentation for the new "reflink" syscall (to create
>> copy-on-write files):
>> http://git.kernel.org/?p=linux/kernel/git/jlbec/ocfs2.git;a=blob;f=Documentation/filesystems/reflink.txt;h=58a6b3879114c0cb591fb736152b30291037edd2;hb=c04db1c934e67337318b177688c0b882297fdd66
>>
>> Have you looked at whether this is close enough to the existing (e.g.
>> Windows NTFS) copy-on-write syntax/feature so that we could use it
>> under Samba (and on the client side from cifs and in the future smb2).
>>   Windows added copy-on-write about 9 or 10 years ago and uses it
>> extensively. although I have not researched enough to see what
>> creating these would look like over the network.
>
>        I didn't know NTFS had it

There are lots of references to the various services that use copy on write
(e.g. the "Groveler" service, which I first noticed back in Windows 2000
copy-on-write linking remote windows install images, which can be quite big,
apparently saving enormous amounts of disk space, and also the "SIS" or
"Single Instance Store" service which is an optional component of Windows
servers, and I saw a reference to Vista using the same NTFS
copy-on-write feature
for some form of backup/snapshot) but this would be a better question
for a Windows programmer not a Linux kernel developer :)

copy-on-write links seem to be represented as "reparse points" (a type of
Windows inode) with a tag "IO_REPARSE_TAG_SIS" (to distinguish it
from other special purpose inodes such as symlinks, DFS referrals and junctions)

http://msdn.microsoft.com/en-us/library/aa365511(VS.85).aspx

Apparently they can be sent over the network based on section 2.4.35 of

http://msdn.microsoft.com/en-us/library/cc232086(PROT.13).aspx
-- 
Thanks,

Steve
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RE: copy-on-write files

[Danilo Almeida]

Joel Becker wrote:

> I didn't know NTFS had it

Copy-on-write is not an NTFS feature.  Rather, it is a feature of the SIS (single instance store) feature that is built on top of NTFS's reparse points.  It consists of a groveller service and a sis.sys filter driver.  The service detects duplicate files and replaces them with SIS reparse points on NTFS which trigger action by the sis.sys filter driver.  For the feature to work on a given volume, the sis.sys filter needs to be active on the volume.

- Danilo