[libvirt] [PATCH 3/3] storage: Adjustments to disk backend to use stateDir

Daniel P. Berrange berrange at redhat.com
Thu Jan 15 15:43:50 UTC 2015

On Wed, Jan 14, 2015 at 07:48:26PM -0500, John Ferlan wrote:
> On 01/14/2015 04:46 AM, Daniel P. Berrange wrote:
> > On Tue, Jan 13, 2015 at 04:12:32PM -0500, John Ferlan wrote:
> >> https://bugzilla.redhat.com/show_bug.cgi?id=1138516
> >>
> >> The disk backend uses 'parted' in order to create and delete partitions
> >> on the disk for use in the pool. At creation time, one can specify a
> >> specific "name" for the volume as well as a specific volume target
> >> format type. The name and volume target type "survive" only as long
> >> as the pool is not refreshed or the libvirtd not restarted/reloaded.
> >>
> >> The action immediately prior to the calling all the backend refreshPool
> >> API's is to clear out all the existing volumes from the pool. The
> >> theory being the refresh will be able to find all elements of the
> >> pool using some mechanism. The disk refreshPool backend will use the
> >> libvirt_parthelper utility to read the partitions found on the disk
> >> in order to regenerate the elements of the pool Unfortunately, since
> >> the "name" and target format type cannot be encoded, the data is now
> >> lost and the defaults are used (for the "name", the partition path
> >> is used and the default of 'none' is used for the target format type).
> >>
> >> This patch solves this by adding the ability to save the XML generated
> >> at create time into the stateDir and then use that during the refreshPool
> >> backend API call to restore the specific fields that are lost.
> > 
> > I don't really think we should be adding a lookaside cache for this.
> Fair enough - although this may then turn into a documentation exercise.
> I'm OK with that option - if that is what is desired.
> > We should simply not permit arbitrary user supplied names for disk
> > based volumes. The names should be required to match the defined
> > naming scheme for disk partitions.  I could have sworn that we had
> > enforced that already when I first wrote this, but perhaps that
> > check got lost somewhere along the way. Likewise for the format - we
> > should just rely on partition table format data
> I looked through some history and didn't see anywhere that supplied name
> must match partition name was enforced, but there's a lot of code motion
> and new features that may cloud the history. Perhaps it's the assumption
> true in so many other pools that 'name' is what was used to create the
> file, directory, lv, etc.  There is code that fills in the name if not
> provided with the partition name (which occurs after Refresh, reload,
> restart).

Looking back some more, it seems I did not ever enforce it - I just
left it as something users had to set the "right way" by convention.
ie I just expected callers to always use the right "sdaNNN" value

> I'm not quite sure how one could enforce a name given that parted
> creates the partition name. One would have to know the device name of
> the pool and the numerical sequence that parted would use (easy perhaps
> know that perhaps partitions 1 & 2 exist that 3 will be next). However,
> there's not necessarily a guarantee of that is there?

For MS-DOS partition tables the partition entry number directly
corresponds to the device name suffix. eg partition 3 corresponds
to /dev/sda3 always and is happy to be sparse. eg if you create
3 partitions and then delete partition 2, you'll end up with
/dev/sda1 and /dev/sda3

Testing with parted the same seems to be true of GPT tables too.

> Reading the libvirt docs - the implication is that the volume "<name>"
> must only be unique across the pool, so the following is "valid":
>     virsh vol-create-as disk-pool vol-linux --format linux 1G
> So without assuming/describing the naming scheme of the underlying
> parted how do we enforce that the name (vol-linux) is the same as what
> parted generates? Or how do we enforce that the name provided ends up
> being the same as the partition created?  It's kind of a chicken/egg issue.

Well with disk partitions we know the device name

eg the pool knows we're operating on '/dev/sda', so when we create
it can we validate that the volume name is '/dev/sdaNNN' by kjust
doing a string prefix match

> Not much is done with the target.format in the backend even though it's
> described as providing the "partition type". When not provided on the
> command line like above it defaults to 'none'. Other than for dos label
> pools that would need to have extended partition before creating a
> logical partition, there's not much "use" for the field other than to
> perhaps store "something" for "someone" (until refresh, reload, restart
> loses it).

When you create a partition you can specify filesystem type for that
partition, even though it is somewhat redundant as it is not directly
connected to the filesystem that you actually mkfs. eg this list of
types from fdisk

Command (m for help): l

 0  Empty           24  NEC DOS         81  Minix / old Lin bf  Solaris        
 1  FAT12           27  Hidden NTFS Win 82  Linux swap / So c1  DRDOS/sec (FAT-
 2  XENIX root      39  Plan 9          83  Linux           c4  DRDOS/sec (FAT-
 3  XENIX usr       3c  PartitionMagic  84  OS/2 hidden C:  c6  DRDOS/sec (FAT-
 4  FAT16 <32M      40  Venix 80286     85  Linux extended  c7  Syrinx         
 5  Extended        41  PPC PReP Boot   86  NTFS volume set da  Non-FS data    
 6  FAT16           42  SFS             87  NTFS volume set db  CP/M / CTOS / .
 7  HPFS/NTFS/exFAT 4d  QNX4.x          88  Linux plaintext de  Dell Utility   
 8  AIX             4e  QNX4.x 2nd part 8e  Linux LVM       df  BootIt         
 9  AIX bootable    4f  QNX4.x 3rd part 93  Amoeba          e1  DOS access     
 a  OS/2 Boot Manag 50  OnTrack DM      94  Amoeba BBT      e3  DOS R/O        
 b  W95 FAT32       51  OnTrack DM6 Aux 9f  BSD/OS          e4  SpeedStor      
 c  W95 FAT32 (LBA) 52  CP/M            a0  IBM Thinkpad hi eb  BeOS fs        
 e  W95 FAT16 (LBA) 53  OnTrack DM6 Aux a5  FreeBSD         ee  GPT            
 f  W95 Ext'd (LBA) 54  OnTrackDM6      a6  OpenBSD         ef  EFI (FAT-12/16/
10  OPUS            55  EZ-Drive        a7  NeXTSTEP        f0  Linux/PA-RISC b
11  Hidden FAT12    56  Golden Bow      a8  Darwin UFS      f1  SpeedStor      
12  Compaq diagnost 5c  Priam Edisk     a9  NetBSD          f4  SpeedStor      
14  Hidden FAT16 <3 61  SpeedStor       ab  Darwin boot     f2  DOS secondary  
16  Hidden FAT16    63  GNU HURD or Sys af  HFS / HFS+      fb  VMware VMFS    
17  Hidden HPFS/NTF 64  Novell Netware  b7  BSDI fs         fc  VMware VMKCORE 
18  AST SmartSleep  65  Novell Netware  b8  BSDI swap       fd  Linux raid auto
1b  Hidden W95 FAT3 70  DiskSecure Mult bb  Boot Wizard hid fe  LANstep        
1c  Hidden W95 FAT3 75  PC/IX           be  Solaris boot    ff  BBT            
1e  Hidden W95 FAT1 80  Old Minix      

I did make a start on defining this for the XML:

 * XXX: these are basically partition types.
 * fdisk has a bazillion partition ID types parted has
 * practically none, and splits the * info across 3
 * different attributes.
 * So this is a semi-generic set
typedef enum {
} virStorageVolFormatDisk;

but then storage_backend_disk never made use of this information

I'm not sure how important this is, but if we do want to make use
of 'format' for the disk pool, this partition types are what I'd
map it too.

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