Re: [NFS] Does "sync" cause the FUA bit to be set?

[Wendy Cheng <s.wendy.cheng@gmail.com>]

Benny Halevy wrote:
> On Jun. 10, 2008, 21:52 +0300, Wendy Cheng <s.wendy.cheng@gmail.com> wrote:
>   
>> Benny Halevy wrote:
>>     
>>> On Jun. 10, 2008, 18:44 +0300, Wendy Cheng <s.wendy.cheng@gmail.com> wrote:
>>>   
>>>       
>>>> Frank Steiner wrote:
>>>>     
>>>>         
>>>>> With the profile ignoring the FUA bit, copying or deleting directories
>>>>> of e.g. 10M with a about 1000 files is factor 5 faster than with the
>>>>> profile honoring the FUA bit.
>>>>>   
>>>>>       
>>>>>           
>>>> FUA bit is normally combined with write-thru scsi command that bypasses 
>>>> storage write cache. I would imagine it needs to well synchronize 
>>>> various pieces before issuing this command. It could hurt the 
>>>> performance if not done well, particularly for meta data. So your result 
>>>> is not surprising.
>>>>     
>>>>         
>>>>> We export with the "sync" option. Does that option maybe set the FUA bit
>>>>> for all write operations on the NFS server?
>>>>>   
>>>>>       
>>>>>           
>>>> It depends on how the filesystem (and its associated disk subsystem) is 
>>>> implemented. The "sync" export option itself has a heavy performance 
>>>> impact, regardless how FUA bit is handled. Some vendors uses specialized 
>>>> HW (e.g. NVRAM) to alleviate this performance hit. If your filesystem 
>>>> doesn't have this type of support, you should expect "sync" option runs 
>>>> much much slower than "async". It is a choice (or balance) between cost, 
>>>> performance, and data reliability.
>>>>     
>>>>         
>>> Wendy, I *think* what you have in mind is the sync mount option
>>> rather than the sync export flag.  The latter just tells the server
>>> not to cheat and do everything asynchronously.  It should *not*
>>> have a heavy performance penalty for I/O intensive writes if the
>>> client is using async writes and commits.
>>>
>>>   
>>>       
>> No, I didn't get confused ... We can use Linux as an example :) .. check 
>> out:
>> http://marc.info/?l=linux-nfs&m=119618886105337&w=2
>>
>> -- quote
>>
>> The default export might have been "async", but unless the option "sync"
>> in /etc/exports was being ignored I was already using "sync". Nevertheless
>> I will try to change to async and test if it makes a difference.
>>
>> (one day later: )
>>
>> I have now tried it and the load on the NFS server is much lower and KDE
>> logins seem to be reasonably fast now.
>>
>> -- un-quote
>>
>>
>> -- Wendy
>>     
>
> I'm not sure how the problem you referred to is related to the
> one at hand.  what I'm trying to say is that the sync _exports_
> flag has stronger affect on namespace modifying workloads, like
> what I believe Frank's workload might be ... [snip]
The referred thread showed "sync" export has performance impacts on 
Linux server *too*.

-- Wendy


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