Re: cephfs file layouts, empty objects in first data pool

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On Sun, Feb 9, 2020 at 3:24 PM Håkan T Johansson <f96hajo@xxxxxxxxxxx>
wrote:

>
> Hi,
>
> running 14.2.6, debian buster (backports).
>
> Have set up a cephfs with 3 data pools and one metadata pool:
> myfs_data, myfs_data_hdd, myfs_data_ssd, and myfs_metadata.
>
> The data of all files are with the use of ceph.dir.layout.pool either
> stored in the pools myfs_data_hdd or myfs_data_ssd.  This has also been
> checked by dumping the ceph.file.layout.pool attributes of all files.
>
> The filesystem has 1617949 files and 36042 directories.
>
> There are however approximately as many objects in the first pool created
> for the cephfs, myfs_data, as there are files.  They also becomes more or
> fewer as files are created or deleted (so cannot be some leftover from
> earlier exercises).  Note how the USED size is reported as 0 bytes,
> correctly reflecting that no file data is stored in them.
>
> POOL_NAME        USED OBJECTS CLONES  COPIES MISSING_ON_PRIMARY UNFOUND
> DEGRADED  RD_OPS      RD   WR_OPS      WR USED COMPR UNDER COMPR
> myfs_data         0 B 1618229      0 4854687                  0       0
>     0 2263590 129 GiB 23312479 124 GiB        0 B         0 B
> myfs_data_hdd 831 GiB  136309      0  408927                  0       0
>     0  106046 200 GiB   269084 277 GiB        0 B         0 B
> myfs_data_ssd  43 GiB 1552412      0 4657236                  0       0
>     0  181468 2.3 GiB  4661935  12 GiB        0 B         0 B
> myfs_metadata 1.2 GiB   36096      0  108288                  0       0
>     0 4828623  82 GiB  1355102 143 GiB        0 B         0 B
>
> Is this expected?
>
> I was assuming that in this scenario, all objects, both their data and any
> keys would be either in the metadata pool, or the two pools where the
> objects are stored.
>
> Is it some additional metadata keys that are stored in the first
> created data pool for cephfs?  This would not be so nice in case the osd
> selection rules for it are using worse disks than the data itself...


https://docs.ceph.com/docs/master/cephfs/file-layouts/#adding-a-data-pool-to-the-mds
notes there is “a small amount of metadata” kept in the primary pool.

That’s not terribly clear; what is actually stored is a per-file location
backtrace (its location in the directory tree) used for hardlink lookups
and disaster recovery scenarios.


>
> Btw: is there any tool to see the amount of key value data size associated
> with a pool?  'ceph osd df' gives omap and meta for osds, but not broken
> down per pool.


I think this is in the newest master code, but I’m not certain which
release it’s in...
-Greg


>
> Best regards,
> Håkan
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>
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