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core.fsyncmethod: add writeout-only mode

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This commit introduces the `core.fsyncMethod` configuration
knob, which can currently be set to `fsync` or `writeout-only`.

The new writeout-only mode attempts to tell the operating system to
flush its in-memory page cache to the storage hardware without issuing a
CACHE_FLUSH command to the storage controller.

Writeout-only fsync is significantly faster than a vanilla fsync on
common hardware, since data is written to a disk-side cache rather than
all the way to a durable medium. Later changes in this patch series will
take advantage of this primitive to implement batching of hardware
flushes.

When git_fsync is called with FSYNC_WRITEOUT_ONLY, it may fail and the
caller is expected to do an ordinary fsync as needed.

On Apple platforms, the fsync system call does not issue a CACHE_FLUSH
directive to the storage controller. This change updates fsync to do
fcntl(F_FULLFSYNC) to make fsync actually durable. We maintain parity
with existing behavior on Apple platforms by setting the default value
of the new core.fsyncMethod option.

Signed-off-by: Neeraj Singh <neerajsi@microsoft.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>
This commit is contained in:
Neeraj Singh
2022-03-10 22:43:20 +00:00
committed by Junio C Hamano
parent 19d3f228c8
commit abf38abec2
13 changed files with 184 additions and 8 deletions
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16
contrib/buildsystems/CMakeLists.txt
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@ -261,10 +261,18 @@ if(CMAKE_SYSTEM_NAME STREQUAL "Windows")
NOGDI OBJECT_CREATION_MODE=1 __USE_MINGW_ANSI_STDIO=0
USE_NED_ALLOCATOR OVERRIDE_STRDUP MMAP_PREVENTS_DELETE USE_WIN32_MMAP
UNICODE _UNICODE HAVE_WPGMPTR ENSURE_MSYSTEM_IS_SET HAVE_RTLGENRANDOM)
list(APPEND compat_SOURCES compat/mingw.c compat/winansi.c compat/win32/path-utils.c
compat/win32/pthread.c compat/win32mmap.c compat/win32/syslog.c
compat/win32/trace2_win32_process_info.c compat/win32/dirent.c
compat/nedmalloc/nedmalloc.c compat/strdup.c)
list(APPEND compat_SOURCES
compat/mingw.c
compat/winansi.c
compat/win32/flush.c
compat/win32/path-utils.c
compat/win32/pthread.c
compat/win32mmap.c
compat/win32/syslog.c
compat/win32/trace2_win32_process_info.c
compat/win32/dirent.c
compat/nedmalloc/nedmalloc.c
compat/strdup.c)
set(NO_UNIX_SOCKETS 1)
elseif(CMAKE_SYSTEM_NAME STREQUAL "Linux")