Re: bcache and SSD over provisioning

[Alex Elsayed <eternaleye-Re5JQEeQqe8AvxtiuMwx3w@public.gmane.org>]

Francis Moreau wrote:

> Hello Matthew,
> 
> Le 13/11/2013 15:13, Matthew Patton a écrit :
>> On Wed, 13 Nov 2013 03:42:54 -0500, Francis Moreau
>> <francis.moro-Re5JQEeQqe8AvxtiuMwx3w@public.gmane.org> wrote:
>> 
>>> First I'd like to know if over-provisioning is still needed when
>>> bcache is used ?
>> 
>> it's needed for any decent, sustained write load if you don't want
>> performance to crash thru the floor. But it doesn't sound like this is
>> your use case. Are you sustaining 5000+ write IOPs?
>> 
> 
> That's indeed not really required: my system is for a desktop usage.
> 
>>>   - Should I create 2 partitions with one left emtpy and whose size is
>>> roughly 7% of the device size ?
>> 
>> Your SSD already has 7% overprovisioning. For best sustained and
>> consistent write performance you'll want to bump it to 25-30%. Yes you
>> just need to partition/LVM down to the desired size.
> 
> Hmm, I don't see what makes you think that my SSD has already 7%
> overprovisioning ? Actually I don't think it's the case.
> 
> Thanks for your help.

Because the raw flash chips are only available in power-of-two sizes. The 
available storage the controller allows access to is 120GB, and it's 
reserved 8GB for overprovisioning if I'm looking at the right model.