Re: SSD usage for bcache - Read and Writeback

[Kai Krakow <hurikhan77@gmail.com>]

Am Thu, 14 Sep 2017 16:45:25 +0200
schrieb Coly Li <i@coly.li>:

> On 2017/9/14 下午3:10, FERNANDO FREDIANI wrote:
> > Hello Coly.
> > 
> > If the users reads a piece of data that is just writen to SSD
> > (unlikely) it should first and in any condition be commited to the
> > permanent storage and then read from there and cached in another
> > area of the SSD. Writaback cache is very volatile and lasts only a
> > few seconds while the data is not yet committed to permanent
> > storage.
> > 
> > In fact multiple device suport is not implemented yet, that's why I
> > am asking it and comparing with other well technology as ZFS.
> >   
> 
> Hi Fernando,
> 
> Do you have some performance number to compare combined and separated
> configurations on ZFS ? If the performance improvement is not from
> adding one more SSD device, I don't why dedicate read/write SSDs may
> help for performance. In my understanding, if any of the SSD has
> spared throughput capability for read or write, mixed them together
> on both SSDs may have better performance number.

I could imagine that one way want to use a fast, more expensive disk as
read cache, while using a smaller SLC SSD as write cache for better
longevity and reliability. Because: When you write cache SSD breaks,
things go really bad. If you read cache breaks: No problem, it just
slows down.

So, in conclusion: The recommendation may not be because of
performance... Better performance may just be a (small) side effect.


> > 
> > On 14/09/2017 04:58, Coly Li wrote:  
> >> On 2017/9/11 下午4:04, FERNANDO FREDIANI wrote:  
>  [...]  
> >> Hi Fernando,
> >>
> >> If there is any real performance number, it will be much easier to
> >> response this idea. What confuses me is, if user reads a data block
> >> which is just written to SSD, what is the benefit for the
> >> separated SSDs.
> >>
> >> Yes I agree with you that some times a single SSD as cache device
> >> is inefficient. Multiple cache device on bcache is a
> >> not-implemented yet feature as I know.
> >>
> >> Thanks.  
> 



-- 
Regards,
Kai

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