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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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11
write-or-die.c
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@@ -62,10 +62,13 @@ void fsync_or_die(int fd, const char *msg)
use_fsync = git_env_bool("GIT_TEST_FSYNC", 1);
if (!use_fsync)
return;
while (fsync(fd) < 0) {
if (errno != EINTR)
die_errno("fsync error on '%s'", msg);
}
if (fsync_method == FSYNC_METHOD_WRITEOUT_ONLY &&
git_fsync(fd, FSYNC_WRITEOUT_ONLY) >= 0)
return;
if (git_fsync(fd, FSYNC_HARDWARE_FLUSH) < 0)
die_errno("fsync error on '%s'", msg);
}
void write_or_die(int fd, const void *buf, size_t count)