Although off_t permits up to 63 bits (8EB) of file offsets, in
practice, we're going to hit other limits first. Document some
of those limits in the qcow2 spec (some are inherent, others are
implementation choices of qemu), and how choice of cluster size
can influence some of the limits.
While we cannot map any uncompressed virtual cluster to any
address higher than 64 PB (56 bits) (due to the current L1/L2
field encoding stopping at bit 55), qemu's cap of 8M for the
refcount table can still access larger host addresses for some
combinations of large clusters and small refcount_order. For
comparison, ext4 with 4k blocks caps files at 16PB.
Another interesting limit: for compressed clusters, the L2 layout
requires an ever-smaller maximum host offset as cluster size gets
larger, down to a 512 TB maximum with 2M clusters. In particular,
note that with a cluster size of 8k or smaller, the L2 entry for
a compressed cluster could technically point beyond the 64PB mark,
but when you consider that with 8k clusters and refcount_order = 0,
you cannot access beyond 512T without exceeding qemu's limit of an
8M cap on the refcount table, it is unlikely that any image in the
wild has attempted to do so. To be safe, let's document that bits
beyond 55 in a compressed cluster must be 0.
Signed-off-by: Eric Blake <eblake@redhat.com>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>
Signed-off-by: Ville Skyttä <ville.skytta@iki.fi>
Reviewed-by: Peter Maydell <peter.maydell@linaro.org>
Reviewed-by: Eric Blake <eblake@redhat.com>
Message-id: 20180612065150.21110-1-ville.skytta@iki.fi
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Compressed clusters are not supposed to have the COPIED bit set, but
this is not made explicit in the specs, so let's document it.
Signed-off-by: Alberto Garcia <berto@igalia.com>
Message-id: 74552e1d6e858d3159cb0c0e188e80bc9248e337.1523376013.git.berto@igalia.com
Reviewed-by: Eric Blake <eblake@redhat.com>
Signed-off-by: Max Reitz <mreitz@redhat.com>
This patch fixes several mistakes in the documentation of the
compressed cluster descriptor:
1) the documentation claims that the cluster descriptor contains the
number of sectors used to store the compressed data, but what it
actually contains is the number of sectors *minus one* or, in other
words, the number of additional sectors after the first one.
2) the width of the fields is incorrectly specified. The number of bits
used by each field is
x = 62 - (cluster_bits - 8) for the offset field
y = (cluster_bits - 8) for the size field
So the offset field's location is [0, x-1], not [0, x] as stated.
3) the size field does not contain the size of the compressed data,
but rather the number of sectors where that data is stored. The
compressed data starts at the exact point specified in the offset
field and ends when there's enough data to produce a cluster of
decompressed data. Both points can be in the middle of a sector,
allowing several compressed clusters to be stored next to one
another, sharing sectors if necessary.
Cc: qemu-stable@nongnu.org
Signed-off-by: Alberto Garcia <berto@igalia.com>
Reviewed-by: Eric Blake <eblake@redhat.com>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>
A bitmap directory entry is sometimes called a 'bitmap header'. This
patch leaves only one name - 'bitmap directory entry'. The name 'bitmap
header' creates misunderstandings with 'qcow2 header' and 'qcow2 bitmap
header extension' (which is extension of qcow2 header)
Signed-off-by: Vladimir Sementsov-Ogievskiy <vsementsov@virtuozzo.com>
Reviewed-by: Eric Blake <eblake@redhat.com>
Reviewed-by: John Snow <jsnow@redhat.com>
Message-id: 20170628120530.31251-3-vsementsov@virtuozzo.com
Signed-off-by: Max Reitz <mreitz@redhat.com>
Signed-off-by: Vladimir Sementsov-Ogievskiy <vsementsov@virtuozzo.com>
Reviewed-by: John Snow <jsnow@redhat.com>
Message-id: 20170628120530.31251-2-vsementsov@virtuozzo.com
Signed-off-by: Max Reitz <mreitz@redhat.com>
Update the qcow2 specification to describe how the LUKS header is
placed inside a qcow2 file, when using LUKS encryption for the
qcow2 payload instead of the legacy AES-CBC encryption
Reviewed-by: Eric Blake <eblake@redhat.com>
Reviewed-by: Alberto Garcia <berto@igalia.com>
Reviewed-by: Max Reitz <mreitz@redhat.com>
Signed-off-by: Daniel P. Berrange <berrange@redhat.com>
Message-id: 20170623162419.26068-13-berrange@redhat.com
Signed-off-by: Max Reitz <mreitz@redhat.com>
This is for the future interoperability & management guide. It includes
the QAPI docs, including the automatically generated ones, other socket
protocols (vhost-user, VNC), and the qcow2 file format.
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
