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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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24
git-compat-util.h
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@ -1271,6 +1271,30 @@ __attribute__((format (printf, 1, 2))) NORETURN
void BUG(const char *fmt, ...);
#endif
#ifdef __APPLE__
#define FSYNC_METHOD_DEFAULT FSYNC_METHOD_WRITEOUT_ONLY
#else
#define FSYNC_METHOD_DEFAULT FSYNC_METHOD_FSYNC
#endif
enum fsync_action {
FSYNC_WRITEOUT_ONLY,
FSYNC_HARDWARE_FLUSH
};
/*
* Issues an fsync against the specified file according to the specified mode.
*
* FSYNC_WRITEOUT_ONLY attempts to use interfaces available on some operating
* systems to flush the OS cache without issuing a flush command to the storage
* controller. If those interfaces are unavailable, the function fails with
* ENOSYS.
*
* FSYNC_HARDWARE_FLUSH does an OS writeout and hardware flush to ensure that
* changes are durable. It is not expected to fail.
*/
int git_fsync(int fd, enum fsync_action action);
/*
* Preserves errno, prints a message, but gives no warning for ENOENT.
* Returns 0 on success, which includes trying to unlink an object that does