[pve-devel] qcow2 bench and speed optimisations

Alexandre DERUMIER aderumier at odiso.com
Fri Sep 14 09:03:51 CEST 2012


more infos:
http://balau82.wordpress.com/2011/05/08/qemu-raw-images-real-size/

so raw and qcow2 with metadatas alloc are sparse files, but you need a filesystem which support it.

The size of file is full size, but it's empty. So filesystem which handle this can by exemple, create 2 files of 20GB on top of a filesystem of 20GB.

I have verified that on my netapp nfs and also ext4.



----- Mail original ----- 

De: "Alexandre DERUMIER" <aderumier at odiso.com> 
À: "Dietmar Maurer" <dietmar at proxmox.com> 
Cc: pve-devel at pve.proxmox.com 
Envoyé: Vendredi 14 Septembre 2012 08:55:05 
Objet: Re: [pve-devel] qcow2 bench and speed optimisations 

>>What performance to you get when you pre-allocate the raw file? 
I don't think we can preallocated a raw file. (the file is empty but have a size of xx giga) 



some doc here from rehdat: 
https://access.redhat.com/knowledge/docs/en-US/Red_Hat_Enterprise_Virtualization/3.0/html/Administration_Guide/Administration_Guide-Manage_Objects.html 

Table 6.3. Permitted Storage Combinations 
Storage Format Type Note 

NFS or iSCSI/FCP Raw or Qcow2 Sparse or Preallocated 
NFS Raw Preallocated A file with an initial size which equals the amount of storage defined for the virtual disk, and has no formatting. 
NFS Raw Sparse A file with an initial size which is close to zero, and has no formatting. 
NFS Qcow2 Sparse A file with an initial size which is close to zero, and has RAW formatting. Subsequent layers will be Qcow2 formatted. 
SAN Raw Preallocated A block device with an initial size which equals the amount of storage defined for the virtual disk, and has no formatting. 
SAN Qcow2 Preallocated A block device with an initial size which equals the amount of storage defined for the virtual disk, and has qcow2 formatting. 
SAN Qcow2 Sparse A block device with an initial size which is much smaller than the size defined for the VDisk (currently 1GB), and has Qcow2 formatting for which space is allocated as needed (currently in 1GB increments). 


note : they seem to use qcow2 on top of iscsi block devices, so it should works with lvm too :) 


----- Mail original ----- 

De: "Dietmar Maurer" <dietmar at proxmox.com> 
À: "Alexandre DERUMIER" <aderumier at odiso.com> 
Cc: pve-devel at pve.proxmox.com 
Envoyé: Vendredi 14 Septembre 2012 07:58:42 
Objet: RE: [pve-devel] qcow2 bench and speed optimisations 

> >>If so, we can do that by default, because metadata is small (128MB/TB). 
> Note that the size file is the full size of the disk (no sparse file). 

That is bad. 

What performance to you get when you pre-allocate the raw file? 
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