Re: bcache compared to more RAM

[Dave Jiang <dave.jiang-ral2JQCrhuEAvxtiuMwx3w@public.gmane.org>]

On Wed 15 Aug 2012 08:29:38 PM MST, Jack Wang wrote:
> 2012/8/16 Dave Jiang <dave.jiang-ral2JQCrhuEAvxtiuMwx3w@public.gmane.org>:
>> On 08/15/2012 04:51 AM, John McCabe-Dansted wrote:
>>> On Wed, Aug 15, 2012 at 7:30 PM, C Sights <csights-97jfqw80gc6171pxa8y+qA@public.gmane.org> wrote:
>>>> Hi all,
>>>>    Has anyone tried creating a bcache device in RAM and benchmarking
>>>> performance of that versus just allowing that RAM to be used as file system
>>>> buffers?
>>> I doubt it; bcache is designed to use non-volatile flash memory rather
>>> than ram.
>>>
>>> If the filesystem is readonly, I imagine that using RAM for bcache
>>> would lead to worse performance as it would just add overhead. If the
>>> filesystem is read-write then it might lead to better performance but
>>> would also lead to the file-system being trashed on if unexpected
>>> power loss occurs.
>>>
>>> Under what circumstance would you intend to actually use ram backed bcache?
>>>
>>
>> So the Intel Xeon E5-2600 series platforms has this feature called
>> asynchronous DRAM refresh (ADR). Essentially it's battery backed DRAM on
>> per DIMM socket basis. I have written a RAM disk driver on top of that
>> experimentally and played a little bit with it used by bcache instead of
>> SSD. Writing to the memory region isn't going to be as fast as normal
>> cached RAM because you have to ensure the data is flushed to the DRAM.
>> However, read should be the same as cached DRAM. But the writing should
>> still be faster than SSD. Anyhow, I'm more curious as to how I can
>> benchmark this to demonstrate if this is a benefit with bcache. i.e.
>> what benchmarking tool(s) I should use and with what parameters etc.
>>
> Hi Dave,
>
> Intesting, wish to see the result. what cache mode is intended to use with ADR.
>
> Jack
> --

Jack,
Right now I'm using wb mode for cache. However when I write I call 
clflush_cache_range() and then read the last dword to make really sure 
that everything has made it to the DRAM. I do wish it's possible on IA 
to just flush without invalidate the cache though.