Re: aio: Question about durability guarantees

[Jeff Moyer <jmoyer@redhat.com>]

Ilya Lantukh <ilantukh@gridgain.com> writes:

> 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?

No.  There are always cases where the I/O submission path can block.  I
find it extremely unusual that you've linked blocking to a particular
type of device, though.  Try running the io_submit.stp systemtap script
to see where you're blocking.

>>> 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!

Please have a look at this article for more information:

https://lwn.net/Articles/457667/

Cheers,
Jeff