Ocfs2 users - Jan 2021 - how to find out the real size of a reflink ?

----- 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
Helmholtz Zentrum M?nchen

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

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
> Helmholtz Zentrum M?nchen
> 
> 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
> 
> 
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