fsync()-in webdav PUT

[Valery Kholodkov]

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On 28-02-18 23:33, Peter Booth wrote:
> This discussion is interesting, educational, and thought provoking.  Web
> architects
> only learn “the right way” by first doing things “the wrong way” and
> seeing what happens.
> Attila and Valery asked questions that sound logical, and I think
> there's value in exploring
> what would happen if their suggestions were implemented.
>
> First caveat - nginx is deployed in all manner different scenarios on
> different hardware
> and operating systems. Physical servers and VMs behave very differently,
> as do local
> and remote storage. When an application writes to NFS mounted storage
> there's no guarantee
> that even and synch will correctly enforce a write barrier. Still, if we
> consider  real numbers:
>
>   * On current model quad socket hosts, nginx can support well over 1
>     million requests per second (see TechEmpower benchmarks)
>   * On the same hardware, a web app that writes to a Postgresql DB can
>     do at least a few thousand writes per second.
>   * A SATA drive might support  300 write IOPS, whilst an SSD will
>     support 100x that.
>
> What this means that doing fully synchronous writes can reduce your
> potential throughput
> by a factor of 100 or more. So it’s not a great way to ensure consistency.
>
> But there are cheaper ways to achieve the same consistency and
> reliability characteristics:
>
>   * If you are using Linux then your reads and write swill occur through
>     the page cache - so the actual disk itself really doesn’t matter
>     (whilst your host is up).
>   * If you want to protect against loss of physical disk then use RAID.
>   * If you want to protect against a random power failure then use
>     drives with battery backed caches, so writes will get persisted when
>     a server restarts after a power failure
>   * If you want to protect against a crazy person hitting your server
>     with an axe then write to two servers ...
>
> *But the bottom line is separation of concerns.* Nginx should not use
> fsync because it isn’t nginx's business.
>
> My two cents,
>
> Peter
>
>
>> On Feb 28, 2018, at 4:41 PM, Aziz Rozyev <arozyev at nginx.com
>> <mailto:arozyev at nginx.com>> wrote:
>>
>> Hello!
>>
>> On Wed, Feb 28, 2018 at 10:30:08AM +0100, Nagy, Attila wrote:
>>
>>> On 02/27/2018 02:24 PM, Maxim Dounin wrote:
>>>>
>>>>> Now, that nginx supports running threads, are there plans to convert at
>>>>> least DAV PUTs into it's own thread(pool), so make it possible to do
>>>>> non-blocking (from nginx's event loop PoV) fsync on the uploaded file?
>>>> No, there are no such plans.
>>>>
>>>> (Also, trying to do fsync() might not be the best idea even in
>>>> threads.  A reliable server might be a better option.)
>>>>
>>> What do you mean by a reliable server?
>>> I want to make sure when the HTTP operation returns, the file is on the
>>> disk, not just in a buffer waiting for an indefinite amount of time to
>>> be flushed.
>>> This is what fsync is for.
>>
>> The question here is - why you want the file to be on disk, and
>> not just in a buffer?  Because you expect the server to die in a
>> few seconds without flushing the file to disk?  How probable it
>> is, compared to the probability of the disk to die?  A more
>> reliable server can make this probability negligible, hence the
>> suggestion.
>>
>> (Also, another question is what "on the disk" meas from physical
>> point of view.  In many cases this in fact means "somewhere in the
>> disk buffers", and a power outage can easily result in the file
>> being not accessible even after fsync().)
>>
>>> Why doing this in a thread is not a good idea? It would'nt block nginx
>>> that way.
>>
>> Because even in threads, fsync() is likely to cause performance
>> degradation.  It might be a better idea to let the OS manage
>> buffers instead.
>>
>> --
>> Maxim Dounin
>> http://mdounin.ru/
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>
>
>
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