Messages in this thread | | | Date | Mon, 19 Nov 2012 20:23:52 -0500 | From | Vladislav Bolkhovitin <> | Subject | Re: [sqlite] light weight write barriers |
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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
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