Re: bcache compared to more RAM

On Wed 15 Aug 2012 08:29:38 PM MST, Jack Wang wrote:

2012/8/16 Dave Jiang [off-list ref]:

quoted

On 08/15/2012 04:51 AM, John McCabe-Dansted wrote:

quoted

On Wed, Aug 15, 2012 at 7:30 PM, C Sights [off-list ref] wrote:

quoted

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.