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[Qemu-devel] [Bug 322602] Re: Snapshot usage makes qcow2 image unusable

From: Thomas Huth
Subject: [Qemu-devel] [Bug 322602] Re: Snapshot usage makes qcow2 image unusable due to large tables
Date: Fri, 04 Nov 2016 15:39:31 -0000

** Changed in: qemu
       Status: New => Incomplete

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  Snapshot usage makes qcow2 image unusable due to large tables

Status in QEMU:

Bug description:
  To reproduce with 0.9.1 and svn:
  - Create a 20G (or some size much greater than system RAM) qcow2 image
  - Inside VM, install some OS, formatting whole drive
  - Create snapshot with savevm
  - Inside VM, reformat and reinstall OS
  - Create snapshot with savevm

  Eventually, qemu crashes, then neither qemu-img nor qemu can open the
  image because memory is exhausted.  The reason is that the whole
  refcount_table is loaded into memory, and this refcount_table has now
  become much bigger than the size of memory due to each snapshot taken
  after significant changes to the disk image.

  The refcount_table really needs to be loaded and used in fixed size
  chunks to avoid this problem.  It will only get worse as typical
  storage set modifications during work sessions between snapshots
  outpace the size of system RAM.

  Alternatively, there needs to be a way to "rollback" a snapshot
  without loading the whole disk image normally, so that a snapshot
  which has made the image unusable in this way can be reversed.

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