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[Qemu-devel] [PATCH V10 4/4] docs: Added MAP_SYNC documentation
From: |
Zhang, Yi |
Subject: |
[Qemu-devel] [PATCH V10 4/4] docs: Added MAP_SYNC documentation |
Date: |
Wed, 23 Jan 2019 11:00:02 +0800 |
From: Zhang Yi <address@hidden>
Signed-off-by: Zhang Yi <address@hidden>
---
docs/nvdimm.txt | 29 ++++++++++++++++++++++++++++-
qemu-options.hx | 4 ++++
2 files changed, 32 insertions(+), 1 deletion(-)
diff --git a/docs/nvdimm.txt b/docs/nvdimm.txt
index 5f158a6..166c395 100644
--- a/docs/nvdimm.txt
+++ b/docs/nvdimm.txt
@@ -142,11 +142,38 @@ backend of vNVDIMM:
Guest Data Persistence
----------------------
+vNVDIMM is designed and implemented to guarantee the guest data
+persistence on the backends in case of host crash or a power failures.
+However, there are still some requirements and limitations
+as explained below.
+
Though QEMU supports multiple types of vNVDIMM backends on Linux,
-currently the only one that can guarantee the guest write persistence
+if MAP_SYNC is not supported by the host kernel and the backends,
+the only backend that can guarantee the guest write persistence
is the device DAX on the real NVDIMM device (e.g., /dev/dax0.0), to
which all guest access do not involve any host-side kernel cache.
+mmap(2) flag MAP_SYNC is added since Linux kernel 4.15. On such
+systems, QEMU can mmap(2) the dax backend files with MAP_SYNC, which
+ensures filesystem metadata consistency in case of a host crash or a power
+failure. Enabling MAP_SYNC in QEMU requires below conditions
+
+ - 'pmem' option of memory-backend-file is 'on':
+ The backend is a file supporting DAX, e.g., a file on an ext4 or
+ xfs file system mounted with '-o dax'. if your pmem=on ,but the backend is
+ not a file supporting DAX, mapping with this flag results in an EOPNOTSUPP
+ error.
+
+ - 'share' option of memory-backend-file is 'on':
+ MAP_SYNC flag available only with the MAP_SHARED_VALIDATE mapping type.
+
+ - 'MAP_SYNC' is supported on linux kernel.(default opened since Linux 4.15)
+
+Otherwise, We will ignore the MAP_SYNC flag.
+
+For more details, please reference mmap(2) man page:
+http://man7.org/linux/man-pages/man2/mmap.2.html.
+
When using other types of backends, it's suggested to set 'unarmed'
option of '-device nvdimm' to 'on', which sets the unarmed flag of the
guest NVDIMM region mapping structure. This unarmed flag indicates
diff --git a/qemu-options.hx b/qemu-options.hx
index 08f8516..0cd41f4 100644
--- a/qemu-options.hx
+++ b/qemu-options.hx
@@ -4002,6 +4002,10 @@ using the SNIA NVM programming model (e.g. Intel NVDIMM).
If @option{pmem} is set to 'on', QEMU will take necessary operations to
guarantee the persistence of its own writes to @option{mem-path}
(e.g. in vNVDIMM label emulation and live migration).
+Also, we will map the backend-file with MAP_SYNC flag, which can ensure
+the file metadata is in sync to @option{mem-path} in case of host crash
+or a power failure. MAP_SYNC requires support from both the host kernel
+(since Linux kernel 4.15) and @option{mem-path} (only files supporting DAX).
@item -object
memory-backend-ram,address@hidden,address@hidden|off},address@hidden|off},address@hidden|off},address@hidden|off},address@hidden,address@hidden,address@hidden|preferred|bind|interleave}
--
2.7.4
- [Qemu-devel] [PATCH V10 0/4] support MAP_SYNC for memory-backend-file, Zhang, Yi, 2019/01/22
- [Qemu-devel] [PATCH V10 4/4] docs: Added MAP_SYNC documentation,
Zhang, Yi <=
- Re: [Qemu-devel] [PATCH V10 4/4] docs: Added MAP_SYNC documentation, Eduardo Habkost, 2019/01/23
- Re: [Qemu-devel] [PATCH V10 4/4] docs: Added MAP_SYNC documentation, Yi Zhang, 2019/01/23
- Re: [Qemu-devel] [PATCH V10 4/4] docs: Added MAP_SYNC documentation, Eduardo Habkost, 2019/01/24
- Re: [Qemu-devel] [PATCH V10 4/4] docs: Added MAP_SYNC documentation, Michael S. Tsirkin, 2019/01/24
- Re: [Qemu-devel] [PATCH V10 4/4] docs: Added MAP_SYNC documentation, Eduardo Habkost, 2019/01/24
- Re: [Qemu-devel] [PATCH V10 4/4] docs: Added MAP_SYNC documentation, Michael S. Tsirkin, 2019/01/24
- Re: [Qemu-devel] [PATCH V10 4/4] docs: Added MAP_SYNC documentation, Eduardo Habkost, 2019/01/24
- Re: [Qemu-devel] [PATCH V10 4/4] docs: Added MAP_SYNC documentation, Yi Zhang, 2019/01/24
- Re: [Qemu-devel] [PATCH V10 4/4] docs: Added MAP_SYNC documentation, Michael S. Tsirkin, 2019/01/24
- Re: [Qemu-devel] [PATCH V10 4/4] docs: Added MAP_SYNC documentation, Eduardo Habkost, 2019/01/24