Re: [MooseFS-Users] Difference between mfsmakesnapshot and mfsappendchunks?

[Piotr R. K. <pio...@mo...>]

Dear William,

> Does mfsappendchunks work the same way? (with the ability to merge multiple files)
> 
Almost.

mfsappendchunks (equivalent of mfssnapshot from MooseFS 1.5) appends a lazy copy of specified file(s) to specified snapshot file ("lazy" means that creation of new chunks is delayed to the moment one copy is modified).  If multiple files are given, they are merged into one target file in the way that each file begins at chunk (64MB) boundary; padding space is left empty.

mfsmakesnapshot  makes a "real" snapshot (lazy copy, like in case of mfsappendchunks) of some object(s) or subtree (similarly to cp -r command). It's atomic with respect to each SOURCE argument separately.  If DESTINATION points to already existing file, error will be reported unless -o (overwrite) option is given. Note: if SOURCE is a directory, it's copied as a whole; but if it's followed by trailing slash, only directory content is copied.


Please find the example below. As you can see, after mfsappendchunks, file “a” have length over 64M. It is because first chunk is filled up with zeroes:

root@ts10:/mnt/mfs/mfsappendchunks# echo "A" > a
root@ts10:/mnt/mfs/mfsappendchunks# echo "B" > b
root@ts10:/mnt/mfs/mfsappendchunks# ls -l
total 1
-rw-r--r-- 1 root root 2 Sep 24 12:25 a
-rw-r--r-- 1 root root 2 Sep 24 12:25 b
root@ts10:/mnt/mfs/mfsappendchunks# mfsfileinfo a
a:
	chunk 0: 0000000002912925_00000001 / (id:43067685 ver:1)
		copy 1: 10.10.10.6:9422 (status:VALID)
		copy 2: 10.10.10.7:9422 (status:VALID)
root@ts10:/mnt/mfs/mfsappendchunks# mfsfileinfo b
b:
	chunk 0: 0000000002912926_00000001 / (id:43067686 ver:1)
		copy 1: 10.10.10.4:9422 (status:VALID)
		copy 2: 10.10.10.10:9422 (status:VALID)
root@ts10:/mnt/mfs/mfsappendchunks# mfsappendchunks a b
root@ts10:/mnt/mfs/mfsappendchunks# mfsfileinfo a
a:
	chunk 0: 0000000002912925_00000001 / (id:43067685 ver:1)
		copy 1: 10.10.10.6:9422 (status:VALID)
		copy 2: 10.10.10.7:9422 (status:VALID)
	chunk 1: 0000000002912926_00000001 / (id:43067686 ver:1)
		copy 1: 10.10.10.4:9422 (status:VALID)
		copy 2: 10.10.10.10:9422 (status:VALID)
root@ts10:/mnt/mfs/mfsappendchunks# mfsfileinfo b
b:
	chunk 0: 0000000002912926_00000001 / (id:43067686 ver:1)
		copy 1: 10.10.10.4:9422 (status:VALID)
		copy 2: 10.10.10.10:9422 (status:VALID)
root@ts10:/mnt/mfs/mfsappendchunks# hexdump -C a
00000000  41 0a 00 00 00 00 00 00  00 00 00 00 00 00 00 00  |A...............|
00000010  00 00 00 00 00 00 00 00  00 00 00 00 00 00 00 00  |................|
*
04000000  42 0a                                             |B.|
04000002
root@ts10:/mnt/mfs/mfsappendchunks# ls -lha
total 70M
drwxr-xr-x  2 root root 2.0M Sep 24 12:25 .
drwxr-xr-x 12 root root 3.9M Sep 24 12:19 ..
-rw-r--r--  1 root root  65M Sep 24 12:26 a
-rw-r--r--  1 root root    2 Sep 24 12:25 b
root@ts10:/mnt/mfs/mfsappendchunks#



Best regards,

-- 
Piotr Robert Konopelko
MooseFS Technical Support Engineer | moosefs.com <https://moosefs.com/>
> On 19 Sep 2015, at 5:13 am, William Kern <wk...@bn...> wrote:
> 
> So I imagine that  
> mfsmakesnapshot is really just cloning the metadata and then fills in after the fact when the source changes.
> Does mfsappendchunks work the same way? (with the ability to merge multiple files)
> Is there an advantage to one method or the other (especially in regards to a VM image)?
> -wk
> 
>  
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