Thoughts about SSD journal size

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Hi,

I have ordered three 240GB Samsung SM863 SSD to my 3 OSD hosts, each with 4 OSDs, to improve write performance.
When looking at the docs, there is a formula for journal size (osd journal size = {2 * (expected throughput * filestore max sync interval)})
that I intend to use. If I understand this correctly it would in my case be (2*(4*100MB/s)*5 seconds)=4GB journal size if I keep
the default filestore max sync interval of 5 seconds. 
Since the SSDs are 240GB, I plan to use significantly larger journals of maybe 40GB, and with the above logic I would increase filestore max sync interval
to 50 seconds.
Is this the correct way of calculating ? Is there any downsides of having a long filestore max sync interval ?

//Daniel

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