zfs discuss - Nov 2008 - ZFS space efficiency when copying files from another source

We''re considering using an OpenSolaris server as a backup server.  Some
of the servers to be backed up would be Linux and Windows servers, and
potentially Windows desktops as well.  What I had imagined was that we could
copy files over to the ZFS-based server nightly, take a snapshot, and only the
blocks that had changed of the files that were being copied over would be stored
on disk.

What I found was that you can take a snapshot, make a small change to a large
file on a ZFS filesystem, take another snapshot, and you''ll only store
a few blocks extra.  However, if you copy the same file of the same name from
another source to the ZFS filesystem, it doesn''t conserve any blocks. 
To a certain extent, I understand why - when copying a file from another system
(even if it''s the same file or a slightly changed version of the same
file), the filesystem actually does write to every block of the file, which I
guess marks all those blocks as changed.

Is there any way to have ZFS check to realize that in fact the blocks being
copied from another system aren''t different, or that only a few of the
blocks are different?  Perhaps there''s another way to copy the file
across the network that only copies the changed blocks.  I believe rsync can do
this, but some of the servers in question are Windows servers and rsync/cygwin
might not be an option.
-- 
This message posted from opensolaris.org

I think what you define IS dedup. You can search the archieves for  
dedup.

Best regards
Mertol

Sent from a mobile device

Mertol Ozyoney

On 17.Kas.2008, at 18:51, BJ Quinn <bjquinn at seidal.com> wrote:
> We''re considering using an OpenSolaris server as a backup server.
> Some of the servers to be backed up would be Linux and Windows  
> servers, and potentially Windows desktops as well.  What I had  
> imagined was that we could copy files over to the ZFS-based server  
> nightly, take a snapshot, and only the blocks that had changed of  
> the files that were being copied over would be stored on disk.
>
> What I found was that you can take a snapshot, make a small change  
> to a large file on a ZFS filesystem, take another snapshot, and  
> you''ll only store a few blocks extra.  However, if you copy the
same
> file of the same name from another source to the ZFS filesystem, it  
> doesn''t conserve any blocks.  To a certain extent, I understand
why
> - when copying a file from another system (even if it''s the same  
> file or a slightly changed version of the same file), the filesystem  
> actually does write to every block of the file, which I guess marks  
> all those blocks as changed.
>
> Is there any way to have ZFS check to realize that in fact the  
> blocks being copied from another system aren''t different, or that
> only a few of the blocks are different?  Perhaps there''s another
way
> to copy the file across the network that only copies the changed  
> blocks.  I believe rsync can do this, but some of the servers in  
> question are Windows servers and rsync/cygwin might not be an option.
> -- 
> This message posted from opensolaris.org
> _______________________________________________
> zfs-discuss mailing list
> zfs-discuss at opensolaris.org
> http://mail.opensolaris.org/mailman/listinfo/zfs-discuss

On Mon, Nov 17, 2008 at 11:51, BJ Quinn <bjquinn at seidal.com>
wrote:
> I believe rsync can do this, but some of the servers in question are
Windows servers and rsync/cygwin might not be an option.
I''d check to make sure rsync has the correct behavior first, but there
is a Windows-based rsync daemon that has some nice properties (runs as
a service, for example) at [1].  You could use the client half of this
program on the Windows end, with the vanilla rsync daemon on the
Solaris end.

Will

[1]: http://www.aboutmyip.com/AboutMyXApp/DeltaCopy.jsp

Thank you both for your responses.  Let me see if I understand correctly - 

1.  Dedup is what I really want, but it''s not implemented yet.

2.  The only other way to accomplish this sort of thing is rsync (in other
words, don''t overwrite the block in the first place if it''s
not different), and if I''m on Windows, I''ll just have to go
ahead and install rsync on my Windows boxes if I want it to work correctly.

Wmurnane, you mentioned there was a Windows-based rsync daemon.  Did you mean
one other than the cygwin-based version?  I didn''t know of any native
Windows rsync software.
-- 
This message posted from opensolaris.org

On Mon, Nov 17, 2008 at 20:54, BJ Quinn <bjquinn at seidal.com>
wrote:
> 1.  Dedup is what I really want, but it''s not implemented yet.
Yes, as I read it.  greenBytes [1] claims to have dedup on their
system; you might investigate them if you decide rsync won''t work for
your application.
> 2.  The only other way to accomplish this sort of thing is rsync (in other
words, don''t overwrite the block in the first place if it''s
not different), and if I''m on Windows, I''ll just have to go
ahead and install rsync on my Windows boxes if I want it to work correctly.
I believe so, yes.  Other programs may have the same capability, but
rsync by any other name would smell as sweet.
> Wmurnane, you mentioned there was a Windows-based rsync daemon.  Did you
mean one other than the cygwin-based version?  I didn''t know of any
native Windows rsync software.
The link I gave ([2]) contains a version of rsync which is
``self-contained''''---it does use Cygwin libraries, but it
includes its
own copies of the ones it needs.  It''s also nicely integrated with the
Windows management tools, in that it uses a Windows service and
Windows scheduled tasks to do its job rather than re-inventing
circular rolling things everywhere.

Will

[1]: http://www.green-bytes.com/
[2]: http://www.aboutmyip.com/AboutMyXApp/DeltaCopy.jsp

On Mon, Nov 17, 2008 at 3:33 PM, Will Murnane <will.murnane at
gmail.com>wrote:
> On Mon, Nov 17, 2008 at 20:54, BJ Quinn <bjquinn at seidal.com>
wrote:
> > 1.  Dedup is what I really want, but it''s not implemented
yet.
> Yes, as I read it.  greenBytes [1] claims to have dedup on their
> system; you might investigate them if you decide rsync won''t work
for
> your application.
>
> > 2.  The only other way to accomplish this sort of thing is rsync (in
> other words, don''t overwrite the block in the first place if
it''s not
> different), and if I''m on Windows, I''ll just have to go
ahead and install
> rsync on my Windows boxes if I want it to work correctly.
> I believe so, yes.  Other programs may have the same capability, but
> rsync by any other name would smell as sweet.
>
> > Wmurnane, you mentioned there was a Windows-based rsync daemon.  Did
you
> mean one other than the cygwin-based version?  I didn''t know of
any native
> Windows rsync software.
> The link I gave ([2]) contains a version of rsync which is
> ``self-contained''''---it does use Cygwin libraries, but it
includes its
> own copies of the ones it needs.  It''s also nicely integrated with
the
> Windows management tools, in that it uses a Windows service and
> Windows scheduled tasks to do its job rather than re-inventing
> circular rolling things everywhere.
>
>
Rsync:
http://www.nexenta.com/corp/index.php?option=com_content&task=view&id=64&Itemid=85
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Here''s an idea - I understand that I need rsync on both sides if I want
to minimize network traffic.  What if I don''t care about that - the
entire file can come over the network, but I specifically only want rsync to
write the changed blocks to disk.  Does rsync offer a mode like that?
-- 
This message posted from opensolaris.org

On Mon, 24 Nov 2008, BJ Quinn wrote:
> Here''s an idea - I understand that I need rsync on both sides if I
> want to minimize network traffic.  What if I don''t care about that
-
> the entire file can come over the network, but I specifically only 
> want rsync to write the changed blocks to disk.  Does rsync offer a 
> mode like that? -- This message posted from opensolaris.org
My understanding is that the way rsync works, if a file already 
exists, then checksums are computed for ranges of the file, and the 
data is only sent/updated if that range is determined to have changed. 
While you can likely configure rsync to send the whole file, I think 
that it does what you want by default.

This is very easy for you to test for yourself.

Bob
=====================================Bob Friesenhahn
bfriesen at simple.dallas.tx.us, http://www.simplesystems.org/users/bfriesen/
GraphicsMagick Maintainer,    http://www.GraphicsMagick.org/

Bob Friesenhahn wrote:
> On Mon, 24 Nov 2008, BJ Quinn wrote:
>
>   
>> Here''s an idea - I understand that I need rsync on both sides
if I
>> want to minimize network traffic.  What if I don''t care about
that -
>> the entire file can come over the network, but I specifically only 
>> want rsync to write the changed blocks to disk.  Does rsync offer a 
>> mode like that? -- This message posted from opensolaris.org
>>     
>
> My understanding is that the way rsync works, if a file already 
> exists, then checksums are computed for ranges of the file, and the 
> data is only sent/updated if that range is determined to have changed. 
> While you can likely configure rsync to send the whole file, I think 
> that it does what you want by default.
>
> This is very easy for you to test for yourself.
>
> Bob
> =====================================> Bob Friesenhahn
> bfriesen at simple.dallas.tx.us,
http://www.simplesystems.org/users/bfriesen/
> GraphicsMagick Maintainer,    http://www.GraphicsMagick.org/
>
> _______________________________________________
> zfs-discuss mailing list
> zfs-discuss at opensolaris.org
> http://mail.opensolaris.org/mailman/listinfo/zfs-discuss
>   
This is indeed the default mode for rsync (deltas only).  The
''-W''
option forces copy of the entire file, rather than just the changes. I 
_believe_ the standard checksum block size is 4kb, but I''m not really 
sure. (it''s buried in the documentation somewhere, and it customizable 
via the -B option)

One note here for ZFS users:

On ZFS (or any other COW filesystem), rsync unfortunately does NOT do 
the "Right Thing" when syncing an existing file.  From ZFS''s
standpoint,
the most efficient way would be merely to rewrite the changed blocks, 
thus allowing COW and snapshots to make a fully efficient storage of the 
changed file.

Unfortunately, rsync instead writes the ENTIRE file to an temp file ( 
.blahtmpfoosomethingorother ) in the same directory as the changed file, 
writes the changed blocks in that copy, then unlinks the original file 
and changes the name to the temp file to the original one.

This results in about worst-case space usage.  I have this problem with 
storing backups of mbox files (don''t ask) - I have large files which 
change frequently, but less than 10% of the file actually changes 
daily.  Due to the way rsync works, ZFS snapshots don''t help me on 
replicated data, so I end up having to restore the entire file every time.

I _really_ wish rsync had an option to "copy in place" or something
like
that, where the updates are made directly to the file, rather than a 
temp copy.




-- 
Erik Trimble
Java System Support
Mailstop:  usca22-123
Phone:  x17195
Santa Clara, CA

On Mon, 24 Nov 2008, Erik Trimble wrote:
>
> One note here for ZFS users:
>
> On ZFS (or any other COW filesystem), rsync unfortunately does NOT do the 
> "Right Thing" when syncing an existing file.  From ZFS''s
standpoint, the most
> efficient way would be merely to rewrite the changed blocks, thus allowing 
> COW and snapshots to make a fully efficient storage of the changed file.
Bummer. In that case, someone should file a bug in rsync''s bug tracker 
(same one as used by Samba) to offer a better ("direct overwrite") 
mode for ZFS.

Bob
=====================================Bob Friesenhahn
bfriesen at simple.dallas.tx.us, http://www.simplesystems.org/users/bfriesen/
GraphicsMagick Maintainer,    http://www.GraphicsMagick.org/

On Mon, Nov 24, 2008 at 08:43:18AM -0800, Erik Trimble
wrote:
> I _really_ wish rsync had an option to "copy in place" or
something like
> that, where the updates are made directly to the file, rather than a 
> temp copy.
Isn''t this what --inplace does?

-- 
albert chin (china at thewrittenword.com)

Rsync can update in-place.   From rsync(1):
            --inplace               update destination files in-place
-- 
This message posted from opensolaris.org

Al Tobey wrote:
> Rsync can update in-place.   From rsync(1):
>             --inplace               update destination files in-place
>   
Whee!  This is now newly working (for me).  I''ve been using an older 
rsync, where this option didn''t work properly on ZFS.

It looks like this was fixed on newer rsync releases.

--inplace does indeed work correctly, at least in the 3.0.4 version I 
just tested on Cygwin.

I''m going to test the 2.6.9 rsync on a Nevada machine right now.

(ok, tested it).  2.6.9 works as expected with --inplace   I suspect 
that the fix in 2.6.4 to --inplace also made it work with ZFS.

Yipee!

-- 
Erik Trimble
Java System Support
Mailstop:  usca22-123
Phone:  x17195
Santa Clara, CA

Oh.  Yup, I had figured this out on my own but forgot to post back.  --inplace
accomplishes what we''re talking about.  --no-whole-file is also
necessary if copying files locally (not over the network), because rsync does
default to only copying changed blocks, but it overrides that default behavior
when not copying over the network.

Also, has anyone figured out a best-case blocksize to use with rsync?  I tried
zfs get volblocksize [pool], but it just returns "-".
-- 
This message posted from opensolaris.org

BJ Quinn wrote:
> Oh.  Yup, I had figured this out on my own but forgot to post back. 
--inplace accomplishes what we''re talking about.  --no-whole-file is
also necessary if copying files locally (not over the network), because rsync
does default to only copying changed blocks, but it overrides that default
behavior when not copying over the network.
> 
> Also, has anyone figured out a best-case blocksize to use with rsync?  I
tried zfs get volblocksize [pool], but it just returns "-".
zfs get recordsize <dataset>


-- 
Darren J Moffat

Should I set that as rsync''s block size?
-- 
This message posted from opensolaris.org