[pve-devel] [PATCH qemu-server] add discard_granularity to 4M for rbd storage

[Alwin Antreich]

On Thu, Jun 28, 2018 at 10:08:45AM +0200, Alexandre Derumier wrote:
> when we have snapshots on rbd and do a trim, the space is increasing
> http://tracker.ceph.com/issues/18352
> 
> we need to trim a full object (4MB by default), to be able to free space.
> 
> test:
> 
> without discard_granularity
> ---------------------------
> vm-107-disk-1		                                       20480M   2500M
> vm-107-disk-1                                                  20480M   2500M
> 
> vm-107-disk-1 at snap1                                            20480M   2500M
> vm-107-disk-1                                                  20480M  90112k
> 
> vm-107-disk-1 at snap1                                            20480M   2500M
> vm-107-disk-1                                                  20480M   1020M
> 
> with discard_granularity=4M
> ---------------------------
> vm-107-disk-1		                                       20480M   2500M
> vm-107-disk-1                                                  20480M   2500M
> 
> vm-107-disk-1 at snap1                                            20480M   2500M
> vm-107-disk-1                                                  20480M  90112k
> 
> vm-107-disk-1 at snap1                                            20480M   2500M
> vm-107-disk-1                                                  20480M    144M
> ---
>  PVE/QemuServer.pm | 5 +++++
>  1 file changed, 5 insertions(+)
> 
> diff --git a/PVE/QemuServer.pm b/PVE/QemuServer.pm
> index 6a355f8..fd9754c 100644
> --- a/PVE/QemuServer.pm
> +++ b/PVE/QemuServer.pm
> @@ -1695,6 +1695,11 @@ sub print_drivedevice_full {
>  	$device .= ",serial=$serial";
>      }
>  
> +    my $volid = $drive->{file};
> +    if($volid && $drive->{discard}) {
> +        my $storage_name = PVE::Storage::parse_volume_id($volid);
> +        $device .= ",discard_granularity=4194304" if $storecfg->{ids}->{$storage_name}->{type} eq 'rbd';
> +    }
In my opinion it would be better to have Qemu figure it out automagically or
use the discard config option to add the granularity.

As example: discard=on / discard=4194304 (is on with granularity).

This way it is configurable per disk image and can be set according to the
needs of the storage (eg. zvol; 8K).

For Ceph, the object size can be set when an image is created (--object-size).

>  
>      return $device;
>  }
> -- 
> 2.11.0
--
Cheers,
Alwin