[Ocfs2-users] how to find out the real size of a reflink ?

[Gang He]

Hi Lentes,

see my comments inline.

On 2021/1/13 21:58, Lentes, Bernd wrote:
> 
> ----- On Jan 13, 2021, at 3:42 AM, Gang He ghe at suse.com wrote:
> 
>> Hi Lentes,
>>
>> you can use "o2info --filestat yourfile" to identify how many data
>> clusters for a cloned file are shared with the original file.
>> Of course, the cloned file does not share meta-data blocks with the
>> original file, only shares the data clusters.
>> e.g.
>> ghe-nd1:/rear/test # o2info --filestat dd.1.ref
>>    File: dd.1.ref
>>    Size: 8589934592      Blocks: 16777216   IO Block: 4096   regular file
>> Device: fe00h/65024d    Inode: 355844      Links: 1
>>   Frag%: 0.01            Clusters: 2097152  Extents: 157    Score: 2
>> Shared: 2097152         Unwritten: 0       Holes: 0        Xattr: 0
>> Access: (0644/-rw-r--r--)  Uid: (    0/    root)   Gid: (    0/    root)
>> Access: 2021-01-13 10:08:06.561968304 +0800
>> Modify: 2021-01-13 10:07:52.74642661
>> Change: 2021-01-13 10:07:52.77958163
>>
>> Please refer to "Shared: 2097152" item.
>>
> 
> Hi Gang,
> 
> thanks for your information.
> 
> i did it:
> 
> ha-idg-1:/mnt/ocfs2 # o2info --filestat reflink/vm_amok.raw
>    File: reflink/vm_amok.raw
>    Size: 880469343744    Blocks: 1719666688 IO Block: 4096   regular file
> Device: fe13h/65043d    Inode: 583953      Links: 1
>   Frag%: 6.60            Clusters: 839681   Extents: 55431  Score: 7
> Shared: 839681          Unwritten: 0       Holes: 0        Xattr: 0
> Access: (0644/-rw-r--r--)  Uid: (    0/    root)   Gid: (    0/    root)
> Access: 2021-01-12 23:18:07.920405221 +0100
> Modify: 2021-01-12 22:40:07.806758460 +0100
> Change: 2021-01-12 22:40:08.404660948 +0100
> ha-idg-1:/mnt/ocfs2 #
> 
> ha-idg-1:/mnt/ocfs2 # o2info --filestat vm_amok.raw
>    File: vm_amok.raw
>    Size: 880469343744    Blocks: 1719666688 IO Block: 4096   regular file
> Device: fe13h/65043d    Inode: 8257821     Links: 1
>   Frag%: 6.60            Clusters: 839681   Extents: 55421  Score: 7
> Shared: 836667          Unwritten: 0       Holes: 0        Xattr: 0
> Access: (0644/-rw-r--r--)  Uid: (  488/srvGeoClue)   Gid: (  487/     ntp)
> Access: 2021-01-13 14:29:43.496432144 +0100
> Modify: 2021-01-13 14:48:02.745794317 +0100
> Change: 2021-01-13 14:48:02.745794317 +0100
> 
> Some questions:
> 
> What is a cluster ?
> What is an extent ?
In the traditional file system, a file consists of meta-data blocks and 
data blocks, but for ocfs2 file system, a file consists of meta-data 
blocks and data clusters, the cluster size can greater than the block 
size to save meta-data block consumption.
The file data is contained by clusters, the file meta data is contained 
by blocks, there are few kind of meta-data blocks, e.g. extent blocks 
looks like to store cluster pointers.

> Shouldn't the number of the shared clusters be the same for both files ?
Yes, when a file is cloned from the original file, both files usually 
own the same number of shared clusters.
But one of files is written again, it's shared clusters should be 
reduced, since the file system have allocated some new clusters for 
these over-write.

Thanks
Gang

> During runtime, the number of shared clusters for the reflink stays the same,
> for the original file it's reducing. Is that expected behaviour ?
> 
> Bernd
> Helmholtz Zentrum München
> 
> Helmholtz Zentrum Muenchen
> Deutsches Forschungszentrum fuer Gesundheit und Umwelt (GmbH)
> Ingolstaedter Landstr. 1
> 85764 Neuherberg
> www.helmholtz-muenchen.de
> Aufsichtsratsvorsitzende: MinDir.in Prof. Dr. Veronika von Messling
> Geschaeftsfuehrung: Prof. Dr. med. Dr. h.c. Matthias Tschoep, Kerstin Guenther
> Registergericht: Amtsgericht Muenchen HRB 6466
> USt-IdNr: DE 129521671
>