fsync()-in webdav PUT

[Peter Booth]

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> 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/ <http://mdounin.ru/>
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