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Deprecate pmem=on with non-DAX capable backend file

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It is not safe to pretend that emulated NVDIMM supports
persistence while backend actually failed to enable it
and used non-persistent mapping as fall back.
Instead of falling-back, QEMU should be more strict and
error out with clear message that it's not supported.
So if user asks for persistence (pmem=on), they should
store backing file on NVDIMM.

Signed-off-by: Igor Mammedov <imammedo@redhat.com>
Reviewed-by: Philippe Mathieu-Daudé <philmd@redhat.com>
Message-Id: <20210111203332.740815-1-imammedo@redhat.com>
Signed-off-by: Eduardo Habkost <ehabkost@redhat.com>
This commit is contained in:
Igor Mammedov 2021-01-11 15:33:32 -05:00 • committed by Eduardo Habkost
parent 80cc1a0dd1
commit cdcf766d0b
2 changed files with 20 additions and 0 deletions
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18
docs/system/deprecated.rst
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@ -221,6 +221,24 @@ This machine is deprecated because we have enough AST2500 based OpenPOWER
machines. It can be easily replaced by the ``witherspoon-bmc`` or the
``romulus-bmc`` machines.
Backend options
---------------
Using non-persistent backing file with pmem=on (since 6.1)
''''''''''''''''''''''''''''''''''''''''''''''''''''''''''
This option is used when ``memory-backend-file`` is consumed by emulated NVDIMM
device. However enabling ``memory-backend-file.pmem`` option, when backing file
is (a) not DAX capable or (b) not on a filesystem that support direct mapping
of persistent memory, is not safe and may lead to data loss or corruption in case
of host crash.
Options are:
- modify VM configuration to set ``pmem=off`` to continue using fake NVDIMM
(without persistence guaranties) with backing file on non DAX storage
- move backing file to NVDIMM storage and keep ``pmem=on``
(to have NVDIMM with persistence guaranties).
Device options
--------------