Re: PostgreSQL's handling of fsync() errors is unsafe and risks data loss at least on XFS

On 18 April 2018 at 05:19, Bruce Momjian <bruce(at)momjian(dot)us> wrote:
> On Tue, Apr 10, 2018 at 05:54:40PM +0100, Greg Stark wrote:
>> On 10 April 2018 at 02:59, Craig Ringer <craig(at)2ndquadrant(dot)com> wrote:
>>
>> > Nitpick: In most cases the kernel reserves disk space immediately,
>> > before returning from write(). NFS seems to be the main exception
>> > here.
>>
>> I'm kind of puzzled by this. Surely NFS servers store the data in the
>> filesystem using write(2) or the in-kernel equivalent? So if the
>> server is backed by a filesystem where write(2) preallocates space
>> surely the NFS server must behave as if it'spreallocating as well? I
>> would expect NFS to provide basically the same set of possible
>> failures as the underlying filesystem (as long as you don't enable
>> nosync of course).
>
> I don't think the write is _sent_ to the NFS at the time of the write,
> so while the NFS side would reserve the space, it might get the write
> request until after we return write success to the process.

It should be sent if you're using sync mode.

From my reading of the docs, if you're using async mode you're already
open to so many potential corruptions you might as well not bother.

I need to look into this more re NFS and expand the tests I have to
cover that properly.

--
Craig Ringer http://www.2ndQuadrant.com/
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