Re: SSD usage for bcache - Read and Writeback

[Kai Krakow <hurikhan77@gmail.com>]

Am Thu, 14 Sep 2017 09:58:25 +0200
schrieb Coly Li <i@coly.li>:

> On 2017/9/11 下午4:04, FERNANDO FREDIANI wrote:
> > Hi folks
> > 
> > In Bcache people normally use a single SSD for both Read and Write
> > cache. This seems to work pretty well, at least for the load we have
> > been using here.
> > 
> > However in other environments, specially on ZFS people tend to
> > suggest to use dedicated SSDs for Write (ZIL) and for Read (L2ARC).
> > Some say that performance will be much better in this way and
> > mainly say they have different wearing levels.
> > The issue now a days is that SSDs for Write Cache (or Writeback)
> > don't need to have much space available (8GB normally is more than
> > enough), just enough for the time until data is committed to the
> > pool (or slower disks) so it is hard to find a suitable SSD to
> > dedicate to this propose only without overprovisioning that part.
> > On the top of that newer SSDs have changed a lot in recent times
> > using different types of memory technologies which tend to be much
> > durable.
> > 
> > Given that I personally see that using a single SSD for both Write
> > and Read cache, in any scenarios doesn't impose any significant
> > loss to the storage, given you use new technology SSDs and that you
> > will hardly saturate it most of the time. Does anyone agree or
> > disagree with that ?  
> 
> 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.

Does bcache support more that one cache device in a cset? If yes, the
best idea would be if one could implement to define one as read-mostly,
and another ssd as write-mostly.

This would make a non-strict policy which allows reading from the other
device if the block is already there, or writing to the read-mostly
device to update data in cache. Thoughts?


-- 
Regards,
Kai

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