Re: [Qemu-devel] [RFC] qed: Add QEMU Enhanced Disk format

[Anthony Liguori]

On 09/07/2010 02:25 PM, Blue Swirl wrote:
> On Mon, Sep 6, 2010 at 10:04 AM, Stefan Hajnoczi
> <address@hidden>  wrote:
>    
> > QEMU Enhanced Disk format is a disk image format that forgoes features
> > found in qcow2 in favor of better levels of performance and data
> > integrity.  Due to its simpler on-disk layout, it is possible to safely
> > perform metadata updates more efficiently.
> >
> > Installations, suspend-to-disk, and other allocation-heavy I/O workloads
> > will see increased performance due to fewer I/Os and syncs.  Workloads
> > that do not cause new clusters to be allocated will perform similar to
> > raw images due to in-memory metadata caching.
> >
> > The format supports sparse disk images.  It does not rely on the host
> > filesystem holes feature, making it a good choice for sparse disk images
> > that need to be transferred over channels where holes are not supported.
> >
> > Backing files are supported so only deltas against a base image can be
> > stored.
> >
> > The file format is extensible so that additional features can be added
> > later with graceful compatibility handling.
> >
> > Internal snapshots are not supported.  This eliminates the need for
> > additional metadata to track copy-on-write clusters.
> >      
> It would be nice to support external snapshots, so another file
> besides the disk images can store the snapshots. Then snapshotting
> would be available even with raw or QED disk images. This is of course
> not QED specific.
>    

There's two types of snapshots that I think can cause confusion.  
There's CPU/device state snapshots and then there's a block device snapshot.

qcow2 and qed both support block device snapshots.  qed only supports 
external snapshots (via backing_file) whereas qcow2 supports external 
and internal snapshots.  The internal snapshots are the source of an 
incredible amount of complexity in the format.

qcow2 can also store CPU/device state snapshots and correlate them to 
block device snapshots (within a single block device).  It only supports 
doing non-live CPU/device state snapshots.

OTOH, qemu can support live snapshotting via live migration.  Today, it 
can be used to snapshot CPU/device state to a file on the filesystem 
with minimum downtime.

Combined with an external block snapshot and correlating data, this 
could be used to implement a single "snapshot" command that would behave 
like savevm but would not pause a guest's execution.

It's really just a matter of plumbing to expose an interface for this 
today.  We have all of the infrastructure we need.

> > + *
> > + * +--------+----------+----------+----------+-----+
> > + * | header | L1 table | cluster0 | cluster1 | ... |
> > + * +--------+----------+----------+----------+-----+
> > + *
> > + * There is a 2-level pagetable for cluster allocation:
> > + *
> > + *                     +----------+
> > + *                     | L1 table |
> > + *                     +----------+
> > + *                ,------'  |  '------.
> > + *           +----------+   |    +----------+
> > + *           | L2 table |  ...   | L2 table |
> > + *           +----------+        +----------+
> > + *       ,------'  |  '------.
> > + *  +----------+   |    +----------+
> > + *  |   Data   |  ...   |   Data   |
> > + *  +----------+        +----------+
> > + *
> > + * The L1 table is fixed size and always present.  L2 tables are allocated on
> > + * demand.  The L1 table size determines the maximum possible image size; it
> > + * can be influenced using the cluster_size and table_size values.
> >      
> The formula for calculating the maximum size would be nice.

table_entries = (table_size * cluster_size / 8)
max_size = (table_entries) * table_entries * cluster_size

it's a hell of a lot easier to do powers-of-two math though:

table_entries = 2^2 * 2^16 / 2^3 = 2^15
max_size = 2^15 * 2^15 * 2^16 = 2^46 = 64TB

>   Is the
> image_size the limit?

No.

>   How many clusters can there be?

table_entries * table_entries

>   What happens if
> the image_size is not equal to multiple of cluster size?

The code checks this and fails at open() or create() time.

>   Wouldn't
> image_size be redundant if cluster_size and table_size determine the
> image size?
>    

In a two level table, if you make table_size the determining factor, the 
image has to be a multiple of the space spanned by the L2 tables which 
in the default case for qed is 2GB.

> > + *
> > + * All fields are little-endian on disk.
> > + */
> > +
> > +typedef struct {
> > +    uint32_t magic;                 /* QED */
> > +
> > +    uint32_t cluster_size;          /* in bytes */
> >      
> Doesn't cluster_size need to be a power of two?
>    

Yes.  It's enforced at open() and create() time but needs to be in the spec.

> > +    uint32_t table_size;            /* table size, in clusters */
> > +    uint32_t first_cluster;         /* first usable cluster */
> >      
> This introduces some limits to the location of first cluster, with 4k
> clusters it must reside within the first 16TB. I guess it doesn't
> matter.
>    

first_cluster is a bad name.  It should be header_size and yeah, there 
is a limit on header_size.

> > +
> > +    uint64_t features;              /* format feature bits */
> > +    uint64_t compat_features;       /* compatible feature bits */
> > +    uint64_t l1_table_offset;       /* L1 table offset, in bytes */
> > +    uint64_t image_size;            /* total image size, in bytes */
> > +
> > +    uint32_t backing_file_offset;   /* in bytes from start of header */
> > +    uint32_t backing_file_size;     /* in bytes */
> > +    uint32_t backing_fmt_offset;    /* in bytes from start of header */
> > +    uint32_t backing_fmt_size;      /* in bytes */
> > +} QEDHeader;
> > +
> > +typedef struct {
> > +    uint64_t offsets[0];            /* in bytes */
> > +} QEDTable;
> >      
> Is this for both L1 and L2 tables?
>    

Yes, which has the nice advantage of simplifying the code quite a bit.

Regards,

Anthony Liguori