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, 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

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