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

[Fabian Grünbichler]

> Fiona Ebner <f.ebner at proxmox.com> hat am 14.05.2025 10:22 CEST geschrieben:
> 
>  
> Am 13.05.25 um 15:31 schrieb Fiona Ebner:
> > Signed-off-by: Fiona Ebner <f.ebner at 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'.

but for ZFS, we actually use the "logical" view provided by `zfs list/get`,
not the "physical" view provided by `zpool list/get` (and even the latter
would already account for redundancy).

e.g., with a testpool consisting of three mirrored vdevs of size 1G, with
a single dataset filled with a file with 512MB of random data:

$ zpool list -v testpool
NAME                 SIZE  ALLOC   FREE  CKPOINT  EXPANDSZ   FRAG    CAP  DEDUP    HEALTH  ALTROOT
testpool             960M   513M   447M        -         -    42%    53%  1.00x    ONLINE  -
  mirror-0           960M   513M   447M        -         -    42%  53.4%      -    ONLINE
    /tmp/vdev1.img     1G      -      -        -         -      -      -      -    ONLINE
    /tmp/vdev2.img     1G      -      -        -         -      -      -      -    ONLINE
    /tmp/vdev3.img     1G      -      -        -         -      -      -      -    ONLINE

and what we use for the storage status:

$ zfs get available,used testpool/data
NAME           PROPERTY   VALUE  SOURCE
testpool/data  available  319M   -
testpool/data  used       512M   -

if we switch away from `stored`, we'd have to account for replication
ourselves to match that, right? and we don't have that information
readily available (and also no idea how to handle EC pools?)? wouldn't
we just exchange one wrong set of numbers with another (differently)
wrong set of numbers?

FWIW, we already provide raw numbers in the pool view, and could maybe
expand that view to provide more details?

e.g., for my test rbd pool the pool view shows 50,29% used amounting to
163,43GiB, whereas the storage status says 51.38% used amounting to
61.11GB of 118.94GB, with the default 3/2 replication

ceph df detail says:

{
      "name": "rbd",
      "id": 2,
      "stats": {
        "stored": 61108710142,               => /1000/1000/1000 == storage used
        "stored_data": 61108699136,
        "stored_omap": 11006,
        "objects": 15579,
        "kb_used": 171373017,
        "bytes_used": 175485968635,          => /1024/1024/1024 == pool used
        "data_bytes_used": 175485935616,
        "omap_bytes_used": 33019,
        "percent_used": 0.5028545260429382,  => rounded this is the pool view percentage
        "max_avail": 57831211008,            => (this + stored)/1000/1000/1000 storage total
        "quota_objects": 0,
        "quota_bytes": 0,
        "dirty": 0,
        "rd": 253354,
        "rd_bytes": 38036885504,
        "wr": 75833,
        "wr_bytes": 33857918976,
        "compress_bytes_used": 0,
        "compress_under_bytes": 0,
        "stored_raw": 183326130176,
        "avail_raw": 173493638191
      }
    },


> 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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