Re: aio: Question about durability guarantees

[Ilya Lantukh <ilantukh@gridgain.com>]

On Tue, Jul 12, 2016 at 3:00 PM, Christoph Hellwig <hch@infradead.org> wrote:
> On Tue, Jul 12, 2016 at 12:42:28PM +0300, Ilya Lantukh wrote:
>> Hi all,
>>
>> We are considering to use libaio in our project as an optional
>> alternative to Java NIO. Our goal is to improve performance while
>> maintaining durability.
>>
>> We have performed tests on different storage devices. Files were
>> opened with flags: O_WRONLY | O_DIRECT | O_DSYNC. However, on some SSD
>> devices O_DSYNC causes io_submit() to block until operation is
>> finished, which is not acceptable in our case.
>
> What is your exact setup?  Sounds like you're using a file system
> and not a block device node.  Which filesystem shows this behavior?

The filesystem is ext4. Yes, we access file using a file system (like
fd = open("~/test/file0", ...);) and for most devices it works fine.
Does it mean that we need to open a block device directly to get full
advantage of aio?

>
>>
>> The question is: do libaio writes with only WRONLY | O_DIRECT flags
>> provide any durability guarantees?
>
> No, they don't, you need O_DSYNC for that.

Got it, thanks!

-- 
Best regards,
Ilya