[PVE-User] Ceph Cache Tiering

[Adam Thompson]

That makes sense - the ZIL acts as a writeback cache for you.  If the latency between QEMU and ZFS were higher (e.g. across NFS, maybe across a router, too) then you'd likely be able to measure a small difference. 
If the latencies are all small enough, any single writeback cache an will likely provide adequate benefit.  If the latencies between layers are large, additional layers of writeback caching *can* help "goodput" but not always (depends on the workload).
-Adam

On October 12, 2016 7:14:53 AM CDT, Lindsay Mathieson <lindsay.mathieson at gmail.com> wrote:
>On 12/10/2016 9:55 PM, Adam Thompson wrote:
>> Ultimately, "Always Use DirectIO" is a religious belief, not a
>technically sound belief.  There are situations where it makes sense,
>and other situations where it doesn't.  The same can be said of*every
>single*  setting - if there were only One True Correct Setting, it
>wouldn't be available as a user-settable option in the first place!
>> -Adam
>
>I wasn't meaning WriteBack or Direct-IO - there is also WriteThrough. I
>
>found with ZFS + SSD SLOG writethrough performance was the same as 
>writeback.
>
>
>But yah, there is no onesize fits all setting ;)
>
>-- 
>Lindsay Mathieson
>
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