From mboxrd@z Thu Jan 1 00:00:00 1970 From: Kai Krakow Subject: Re: SSD usage for bcache - Read and Writeback Date: Thu, 14 Sep 2017 17:31:12 +0200 Message-ID: <20170914173112.376c03ef@jupiter.sol.kaishome.de> References: <51852c79-bee0-19c3-92d8-6044f3e3e2a7@coly.li> <4a5bb8a9-3de9-bae9-abea-cfa014c87084@upx.com> Mime-Version: 1.0 Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 8BIT Return-path: Received: from [195.159.176.226] ([195.159.176.226]:33467 "EHLO blaine.gmane.org" rhost-flags-FAIL-FAIL-OK-OK) by vger.kernel.org with ESMTP id S1751131AbdINPbW (ORCPT ); Thu, 14 Sep 2017 11:31:22 -0400 Received: from list by blaine.gmane.org with local (Exim 4.84_2) (envelope-from ) id 1dsW6i-00077X-GV for linux-bcache@vger.kernel.org; Thu, 14 Sep 2017 17:31:12 +0200 Sender: linux-bcache-owner@vger.kernel.org List-Id: linux-bcache@vger.kernel.org To: linux-bcache@vger.kernel.org Am Thu, 14 Sep 2017 16:45:25 +0200 schrieb Coly Li : > On 2017/9/14 下午3:10, FERNANDO FREDIANI wrote: > > Hello Coly. > > > > If the users reads a piece of data that is just writen to SSD > > (unlikely) it should first and in any condition be commited to the > > permanent storage and then read from there and cached in another > > area of the SSD. Writaback cache is very volatile and lasts only a > > few seconds while the data is not yet committed to permanent > > storage. > > > > In fact multiple device suport is not implemented yet, that's why I > > am asking it and comparing with other well technology as ZFS. > > > > Hi Fernando, > > Do you have some performance number to compare combined and separated > configurations on ZFS ? If the performance improvement is not from > adding one more SSD device, I don't why dedicate read/write SSDs may > help for performance. In my understanding, if any of the SSD has > spared throughput capability for read or write, mixed them together > on both SSDs may have better performance number. I could imagine that one way want to use a fast, more expensive disk as read cache, while using a smaller SLC SSD as write cache for better longevity and reliability. Because: When you write cache SSD breaks, things go really bad. If you read cache breaks: No problem, it just slows down. So, in conclusion: The recommendation may not be because of performance... Better performance may just be a (small) side effect. > > > > On 14/09/2017 04:58, Coly Li wrote: > >> On 2017/9/11 下午4:04, FERNANDO FREDIANI wrote: > [...] > >> Hi Fernando, > >> > >> 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. > >> > >> Thanks. > -- Regards, Kai Replies to list-only preferred.