Re: When ceph synchronizes journal to disk?

[Xing Lin <xinglin@cs.utah.edu>]

Thanks very much for all your explanations. I am now much clearer about 
it. Have a great day!

Xing

On 03/05/2013 01:12 PM, Greg Farnum wrote:
>> All the data goes to the disk in write-back mode so it isn't safe yet
>> >until the flush is called. That's why it goes into the journal first, to
>> >be consistent at all times.
>> >  
>> >If you would buffer everything in the journal and flush that at once you
>> >would overload the disk for that time.
>> >  
>> >Let's say you have 300MB in the journal after 10 seconds and you want to
>> >flush that at once. That would mean that specific disk is unable to do
>> >any other operations then writing with 60MB/sec for 5 seconds.
>> >  
>> >It's better to always write in write-back mode to the disk and flush at
>> >a certain point.
>> >  
>> >In the meantime the scheduler can do it's job to balance between the
>> >reads and the writes.
>> >  
>> >Wido
> Yep, what Wido said. Specifically, we do force the data to the journal with an fsync or equivalent before responding to the client, but once it's stable on the journal we give it to the filesystem (without doing any sort of forced sync). This is necessary — all reads are served from the filesystem.
> -Greg

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