Do not acquire lock for file.Sync() fsync call

[1978629634]

In the Flush() function, a lock is acquired before calling file.flush and file.sync. file.flush writes the glog buffer into the kernel buffer, while file.sync writes the kernel buffer to disk.

We encountered a problem where when disk I/O is heavy(the cloud service provider offers poor cloud disk performance), all glog logs would be blocked due to waiting for file.sync.

file.sync ensures that logs are flushed to disk, but I believe that acquiring a lock before calling file.sync is unnecessary. The kernel has an efficient buffered I/O mechanism, and file.sync does not block kernel buffered I/O. We only need to request that logs are flushed to disk and do not need to acquire a lock and wait for the kernel to complete the flush. Acquiring a lock and waiting for logs to be flushed to disk cannot make good use of the kernel's buffered I/O, and it can lead to performance issues.

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[1978629634]

Hi @chressie @stapelberg , Could you please take a look at this pull request when you have a chance? I'd appreciate your review and feedback.

[chressie]

Hi @1978629634.

Thanks for your PR and for taking the time to explain and draft a fix. While the change you're requesting is reasonable, we cannot merge it directly into this repository. We have to make an internal change first, that we'll then be able to push upstream.

Quote from the README:

The repository contains an open source version of the log package used inside Google. The master copy of the source lives inside Google, not here. The code in this repo is for export only and is not itself under development. Feature requests will be ignored.

Send bug reports to [email protected].

I'm closing this PR and will soon merge a new version with a fix. Thanks for your understanding.