Re: [pve-devel] [PATCH storage 2/2] rbd plugin: status: explain why percentage value can be different from Ceph

[Fiona Ebner <f.ebner@proxmox.com>]

Am 13.05.25 um 15:31 schrieb Fiona Ebner:
> Signed-off-by: Fiona Ebner <f.ebner@proxmox.com>
> ---
>  src/PVE/Storage/RBDPlugin.pm | 6 ++++++
>  1 file changed, 6 insertions(+)
> 
> diff --git a/src/PVE/Storage/RBDPlugin.pm b/src/PVE/Storage/RBDPlugin.pm
> index 154fa00..b56f8e4 100644
> --- a/src/PVE/Storage/RBDPlugin.pm
> +++ b/src/PVE/Storage/RBDPlugin.pm
> @@ -703,6 +703,12 @@ sub status {
>  
>      # max_avail -> max available space for data w/o replication in the pool
>      # stored -> amount of user data w/o replication in the pool
> +    # NOTE These values are used because they are most natural from a user perspective.
> +    # However, the %USED/percent_used value in Ceph is calculated from values before factoring out
> +    # replication, namely 'bytes_used / (bytes_used + avail_raw)'. In certain setups, e.g. with LZ4
> +    # compression, this percentage can be noticeably different form the percentage
> +    # 'stored / (stored + max_avail)' shown in the Proxmox VE CLI/UI. See also src/mon/PGMap.cc from
> +    # the Ceph source code, which also mentions that 'stored' is an approximation.
>      my $free = $d->{stats}->{max_avail};
>      my $used = $d->{stats}->{stored};
>      my $total = $used + $free;

Thinking about this again, I don't think continuing to use 'stored' is
best after all, because that is before compression. And this is where
the mismatch really comes from AFAICT. For highly compressible data, the
mismatch between actual usage on the storage and 'stored' can be very
big (in a quick test using the 'yes' command to fill an RBD image, I got
stored = 2 * (used / replication_count)). And here in the storage stats
we are interested in the usage on the storage, not the actual amount of
data written by the user. For ZFS we also don't use 'logicalused', but
'used'.

From src/osd/osd_types.h:

>   int64_t data_stored = 0;                ///< Bytes actually stored by the user
>   int64_t data_compressed = 0;            ///< Bytes stored after compression
>   int64_t data_compressed_allocated = 0;  ///< Bytes allocated for compressed data
>   int64_t data_compressed_original = 0;   ///< Bytes that were compressed



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