Re: [sqlite] light weight write barriers

[Vladislav Bolkhovitin <vst@vlnb.net>]

Chris Friesen, on 11/15/2012 05:35 PM wrote:
>> The easiest way to implement this fsync would involve three things:
>> 1. Schedule writes for all dirty pages in the fs cache that belong to
>> the affected file, wait for the device to report success, issue a cache
>> flush to the device (or request ordering commands, if available) to make
>> it tell the truth, and wait for the device to report success. AFAIK this
>> already happens, but without taking advantage of any request ordering
>> commands.
>> 2. The requesting thread returns as soon as the kernel has identified
>> all data that will be written back. This is new, but pretty similar to
>> what AIO already does.
>> 3. No write is allowed to enqueue any requests at the device that
>> involve the same file, until all outstanding fsync complete [3]. This is
>> new.
>
> This sounds interesting as a way to expose some useful semantics to userspace.
>
> I assume we'd need to come up with a new syscall or something since it doesn't
> match the behaviour of posix fsync().

This is how I would export cache sync and requests ordering abstractions to the 
user space:

For async IO (io_submit() and friends) I would extend struct iocb by flags, which 
would allow to set the required capabilities, i.e. if this request is FUA, or full 
cache sync, immediate [1] or not, ORDERED or not, or all at the same time, per 
each iocb.

For the regular read()/write() I would add to "flags" parameter of 
sync_file_range() one more flag: if this sync is immediate or not.

To enforce ordering rules I would add one more command to fcntl(). It would make 
the latest submitted write in this fd ORDERED.

All together those should provide the requested functionality in a simple, 
effective, unambiguous and backward compatible manner.

Vlad

1. See my other today's e-mail about what is immediate cache sync.