fsync()-in webdav PUT

Nagy, Attila bra at fsn.hu
Fri Mar 2 09:12:02 UTC 2018

On 02/28/2018 03:08 PM, Maxim Dounin wrote:
> 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.
Because the files I upload to nginx servers are important to me. Please 
step back a little and forget that we are talking about nginx or an HTTP 
We have data which we want to write to somewhere.
Check any of the database servers. Would you accept a DB server which 
can loose confirmed data or couldn't be configured that way that a 
write/insert/update/commit/whatever you use to modify or put data into 
it operation is reliably written by the time you receive acknowledgement?
Now try to use this example. I would like to use nginx to store files. 
That's what HTTP PUT is for.
Of course I'm not expecting that the server will die every day. But when 
that happens, I want to make sure that the confirmed data is there.
Let's take a look at various object storage systems, like ceph. Would 
you accept a confirmed write to be lost there? They make a great deal of 
work to make that impossible.
Now try to imagine that somebody doesn't need the complexity of -for 
example- ceph, but wants to store data with plain HTTP. And you got 
there. If you store data, then you want to make sure the data is there.
If you don't, why do you store it anyways?

> (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().)
Not with good software/hardware. (and it doesn't really have to be super 
good, but average)

>> 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.
Sure, it will cause some (not much BTW in a good configuration). But if 
my primary goal is to store files reliably, why should I care?
I can solve that by using SSDs for logs, BBWCs and a lot more thing. But 
in the current way, I can't make sure that a HTTP PUT was really 
successful or it will be successful in some seconds or it will fail badly.

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