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 13:43:11 +0200 Message-ID: <20170914134311.25fd67aa@jupiter.sol.kaishome.de> References: <51852c79-bee0-19c3-92d8-6044f3e3e2a7@coly.li> 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]:52228 "EHLO blaine.gmane.org" rhost-flags-FAIL-FAIL-OK-OK) by vger.kernel.org with ESMTP id S1751701AbdINLpN (ORCPT ); Thu, 14 Sep 2017 07:45:13 -0400 Received: from list by blaine.gmane.org with local (Exim 4.84_2) (envelope-from ) id 1dsSZr-0007fD-HT for linux-bcache@vger.kernel.org; Thu, 14 Sep 2017 13:45:03 +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 09:58:25 +0200 schrieb Coly Li : > On 2017/9/11 下午4:04, FERNANDO FREDIANI wrote: > > Hi folks > > > > In Bcache people normally use a single SSD for both Read and Write > > cache. This seems to work pretty well, at least for the load we have > > been using here. > > > > However in other environments, specially on ZFS people tend to > > suggest to use dedicated SSDs for Write (ZIL) and for Read (L2ARC). > > Some say that performance will be much better in this way and > > mainly say they have different wearing levels. > > The issue now a days is that SSDs for Write Cache (or Writeback) > > don't need to have much space available (8GB normally is more than > > enough), just enough for the time until data is committed to the > > pool (or slower disks) so it is hard to find a suitable SSD to > > dedicate to this propose only without overprovisioning that part. > > On the top of that newer SSDs have changed a lot in recent times > > using different types of memory technologies which tend to be much > > durable. > > > > Given that I personally see that using a single SSD for both Write > > and Read cache, in any scenarios doesn't impose any significant > > loss to the storage, given you use new technology SSDs and that you > > will hardly saturate it most of the time. Does anyone agree or > > disagree with that ? > > 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. Does bcache support more that one cache device in a cset? If yes, the best idea would be if one could implement to define one as read-mostly, and another ssd as write-mostly. This would make a non-strict policy which allows reading from the other device if the block is already there, or writing to the read-mostly device to update data in cache. Thoughts? -- Regards, Kai Replies to list-only preferred.