Re: [sqlite] light weight write barriers

[Vladislav Bolkhovitin <vst@vlnb.net>]

Vladislav Bolkhovitin, on 11/17/2012 12:02 AM 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.

Correction. To avoid possible races better that the new fcntl() command would 
specify that N subsequent read()/write()/sync() calls as ORDERED.

For instance, in the simplest case of N=1, one next after fcntl() write() would be 
handled as ORDERED.

(Unfortunately, it doesn't look like this old read()/write() interface has space 
for a more elegant solution)

Vlad