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

[Herbert Van Den Bergh]

From man mkfs.ocfs2:

       -C, --cluster-size cluster-size
              Valid cluster size values are 4K, 8K, 16K, 32K, 64K, 128K,  256K,  512K  and
              1M.  If  omitted,  a  value  will  be  heuristically determined based on the
              expected usage of the file system (see the -T option). For volumes  expected
              to  store  large files, like database files, while a cluster size of 128K or
              more is recommended, one can opt for a smaller size as long as that value is
              not smaller than the database block size.  For others, use 4K.

An extent is a series of blocks in a file. When a file grows, its last extent is grown if there are free blocks on disk following the extent, or a new extent is added using free blocks from someplace else on the disk. The filefrag -v command will list all extents of a file, and whether they’re shared with other files or not.

Thanks,
Herbert.


> On Jan 14, 2021, at 1:31 AM, Lentes, Bernd <bernd.lentes at helmholtz-muenchen.de> wrote:
> 
> 
> 
> ----- On Jan 14, 2021, at 2:50 AM, Gang He ghe at suse.com wrote:
> 
>> Hi Lentes,
>> 
>> see my comments inline.
> 
>>> 
>>> 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.
>> 
> 
> What is the size of a cluster ? 1MB ?
> And what is an extent ?
> 
> Thanks.
> 
> Bernd
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