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[Qemu-devel] [RFC] qemu-img: add option -d in convert

From: Wenchao Xia
Subject: [Qemu-devel] [RFC] qemu-img: add option -d in convert
Date: Thu, 20 Jun 2013 16:59:17 +0800
User-agent: Mozilla/5.0 (Windows NT 5.1; rv:17.0) Gecko/20130509 Thunderbird/17.0.6

  This is a draft design which aimed for internal snapshot convert,
hope to get your comments:

  Internal snapshot is not as easy as external snapshot, to query and
convert. This patch will improve convertion side, which helps internal
/ external snapshot mixed case. With it user can treat internal
snapshot as lineraity relationship, use it like external ones with
tool qemu-img.

An detailed example, If there is a chain as following:


The real relationship in it could be:
|->imageA(sn0)   |->imageB(sn0)        |->imageC(sn0)

To export it, two steps:
1. duplicate them to get an exactly same tree by:
qemu-img convert imageA.qcow2 -O export/imageA.qcow2 -f qcow2
qemu-img convert imageA.qcow2 -s sn0 -O export/imageA_sn0.qcow2
qemu-img convert imageB.qcow2 -O export/imageB.qcow2 -f qcow2 -o
qemu-img convert imageB.qcow2 -s sn0 -O export/imageB.qcow2 -f qcow2 -o

result at ./export:
|->imageA_sn0.qcow2  |->imageB_sn0.qcow2   |->imageC_sn0.qcow2

2. change the relationship to linearity to save space(or by 3rd party
diff tool):
qemu-img create imageA_l.qcow2 -f qcow2 -p backing_file=imageA_qcow2
qemu-img rebase imageA_l.qcow2 -b imageA_sn0.qcow2
qemu-img rebase -u imageB.qcow2 -b imageA_l.qcow2
discard imageA.qcow2

result at ./export:

This is a bit complexity, they can be merged into one step, to save
disk I/O and make procedure simple, add a parameter:
[-d [base_image=IMAGE,]snapshot=SNAPSHOT]

qemu-img convert imageA.qcow2 -s sn0 -O export/imageA_sn0.qcow2 -f qcow2
qemu-img convert imageA.qcow2 -d snapshot=sn0 -O export/imageA.qcow2 -f
qcow2 -o backing_file=export/imageA_sn0.qcow2

result at ./export:

parameter base_image allow diff operation taken across image in the
backing chain.

  1 snapshot query can be added in qemu-nbd easily later.
  2 This is actually a work around by qemu-img and qemu-nbd. A better
way is to provide user snapshot_read() and snapshot_allocated()
interface, typically a library. But that need some adjust in block
level, especially thread, coroutine, and emulator cut off, so delay

Best Regards

Wenchao Xia

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