From mboxrd@z Thu Jan 1 00:00:00 1970 From: Brad Walker Subject: Re: problem w/ read caching.. Date: Thu, 27 Sep 2012 23:28:20 +0000 (UTC) Message-ID: References: Mime-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Transfer-Encoding: 7bit Return-path: Sender: linux-bcache-owner-u79uwXL29TY76Z2rM5mHXA@public.gmane.org To: linux-bcache-u79uwXL29TY76Z2rM5mHXA@public.gmane.org List-Id: linux-bcache@vger.kernel.org Kent Overstreet writes: > > Sounds like a good portion of your IO is bypassing the cache. That > will happen if some of it's sequential, or if the SSD latency goes > over a threshold - sequential_cutoff, congested_read_threshold_us and > congest_write_threshold_us (if I'm remembering the names correctly) > are the settings that control all that. 0 disables all of them. > So I set the sequential_cutoff and congested_read_threshold_us to both be 0. Since I was only doing reads, I figured there was no need to mess with the write option. But, I'm still seeing a problem. My hardware is: 1 - Dell PowerEdge R710 w/ 24 x Xeon processors, 96GB of ram 2 - Micron P320H SSD 3 - LSI storage device connected by a SAS interface What I see is that when I do random reads over a 10GB region, the cache warms up but hits a read response plateau at about 7ms. I still see a LOT (i.e. 32000 IOPS) of I/O to the disk. Yes, if I run the same test over a 1GB region, runs really fast. Pretty close to the max IOPS rate of the SSD. So I'm thinking there is a problem here or I have a bcache config issue. I'm willing to try things but I need some guidance on what to look for as it seems like a bcache issue. Thanks for the help. -brad w.