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kbase: incrementalbackupinternals: Replace bash with pseudocode

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Simplify the docs and reduce maintenance burden by just describing the
algorithm by a pseudo-language. Users are encouraged to use libvirt
anyways and projects such as oVirt which do some management of storage
themselves are unlikely to use bash anyways.

Signed-off-by: Peter Krempa <pkrempa@redhat.com>
Reviewed-by: Eric Blake <eblake@redhat.com>
This commit is contained in:
Peter Krempa 2020-06-24 11:41:28 +02:00
parent abf20e65f7
commit 4d064f9787
1 changed files with 11 additions and 33 deletions
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44
docs/kbase/incrementalbackupinternals.rst
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@ -228,42 +228,20 @@ following rules:
Creating external snapshots manually
--------------------------------------
To create the same topology outside of libvirt (e.g when doing snapshots offline)
a new ``qemu-img`` which supports the ``bitmap`` subcommand is recommended. The
following algorithm then ensures that the new image after snapshot will work
with backups (note that ``jq`` is a JSON processor):
To create the same topology outside of libvirt (e.g when doing snapshots
offline) the following pseudo-algorithm ensures that the new image after
snapshot will work with backups. ``OVERLAY`` corresponds to the new overlay
image, ``ACTIVE`` corresponds to the topmost image of the active chain prior to
the snapshot.
::
#!/bin/bash
create image OVERLAY on top of ACTIVE
# arguments
SNAP_IMG="vda-2.qcow2"
BACKING_IMG="vda-1.qcow2"
for each BITMAP in ACTIVE:
let GRANULARITY = granularity of BITMAP in ACTIVE
# constants - snapshots and bitmaps work only with qcow2
SNAP_FMT="qcow2"
BACKING_IMG_FMT="qcow2"
if BITMAP isn't RECORDING or is INCONSISTENT:
continue
# create snapshot overlay
qemu-img create -f "$SNAP_FMT" -F "$BACKING_IMG_FMT" -b "$BACKING_IMG" "$SNAP_IMG"
BACKING_IMG_INFO=$(qemu-img info --output=json -f "$BACKING_IMG_FMT" "$BACKING_IMG")
BACKING_BITMAPS=$(jq '."format-specific".data.bitmaps' <<< "$BACKING_IMG_INFO")
if [ "x$BACKING_BITMAPS" = "xnull" ]; then
exit 0
fi
for BACKING_BITMAP_ in $(jq -c '.[]' <<< "$BACKING_BITMAPS"); do
BITMAP_FLAGS=$(jq -c -r '.flags[]' <<< "$BACKING_BITMAP_")
BITMAP_NAME=$(jq -r '.name' <<< "$BACKING_BITMAP_")
if grep 'in-use' <<< "$BITMAP_FLAGS" ||
grep -v 'auto' <<< "$BITMAP_FLAGS"; then
continue
fi
qemu-img bitmap -f "$SNAP_FMT" "$SNAP_IMG" --add "$BITMAP_NAME"
done
create RECORDING bitmap named BITMAP in OVERLAY with GRANULARITY