From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: X-Spam-Checker-Version: SpamAssassin 3.4.0 (2014-02-07) on aws-us-west-2-korg-lkml-1.web.codeaurora.org X-Spam-Level: X-Spam-Status: No, score=-2.5 required=3.0 tests=DKIM_SIGNED,DKIM_VALID, HEADER_FROM_DIFFERENT_DOMAINS,MAILING_LIST_MULTI,SPF_HELO_NONE,SPF_PASS, URIBL_BLOCKED,USER_AGENT_SANE_1 autolearn=no autolearn_force=no version=3.4.0 Received: from mail.kernel.org (mail.kernel.org [198.145.29.99]) by smtp.lore.kernel.org (Postfix) with ESMTP id 376DEC49ED9 for ; Thu, 12 Sep 2019 09:50:30 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [209.132.180.67]) by mail.kernel.org (Postfix) with ESMTP id D719B20663 for ; Thu, 12 Sep 2019 09:50:29 +0000 (UTC) Authentication-Results: mail.kernel.org; dkim=pass (2048-bit key) header.d=toxicpanda-com.20150623.gappssmtp.com header.i=@toxicpanda-com.20150623.gappssmtp.com header.b="N7HDKonL" Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1730831AbfILJu2 (ORCPT ); Thu, 12 Sep 2019 05:50:28 -0400 Received: from mail-yw1-f66.google.com ([209.85.161.66]:36393 "EHLO mail-yw1-f66.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1730618AbfILJu2 (ORCPT ); Thu, 12 Sep 2019 05:50:28 -0400 Received: by mail-yw1-f66.google.com with SMTP id x64so8913522ywg.3 for ; Thu, 12 Sep 2019 02:50:27 -0700 (PDT) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=toxicpanda-com.20150623.gappssmtp.com; s=20150623; h=date:from:to:cc:subject:message-id:references:mime-version :content-disposition:in-reply-to:user-agent; bh=0WVqWa+CQ6KBO7qcJGccvfEb1mGXQKGcoGeUP40DvXA=; b=N7HDKonLLIbBVljivsZMsYyfzlgAK9RgAMLOUSzC8Bz8bLltXy4sTTg5RjTpLmaMRR idR+2RG4wwBti5mA2+4J3zwl4gp9WvLAm63iHtzCC4QMAckpRGQ1p8XWgROQITUxq6JG GQRAKFSHPK60eCPg/dkl4ndT+5sU+zDeln9idz8SPQ1snER+yvOl5E7AKV6ptKgevdni mLKcheRrX//GEAuTaPFr6gkmqywgFKxEK3TrFPnatrqSYJ86rMBxk2lboUUEG7ZOj0V+ CgZjx1XoN4RJFZaibOvLW+FiTMhU2PlkwnFisZhTW8rEokIg1ES7q56bOzRwbjWlUBi3 b6Fg== X-Google-DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=1e100.net; s=20161025; h=x-gm-message-state:date:from:to:cc:subject:message-id:references :mime-version:content-disposition:in-reply-to:user-agent; bh=0WVqWa+CQ6KBO7qcJGccvfEb1mGXQKGcoGeUP40DvXA=; b=p5plrVHjuPeyFYran7NXUwk3LZ4sTRvVE4D6EnvsE0zR5ieNzLc4Oc58wCXFgs7E91 nDZPLuKlTuacQiy90x/2lTAtTQnidZ1datRK3vYAuv5fnvhzI6HARGbkTYQK079CfHyO uB/pxOpzmX1KUEcnKbWCuITBIQ6P7VenqXEeQ30GnmbxXqMeHk1pWDBRMeZqUQPTQHf0 takq4PGJ+9hwR4VzqCvBCB5gARqD/N2WQjWU3FOql6VFqWXiCu84GKx26BhGewYy/gOH JYojPKKxEJSL5AhDvXO8AKdyQLG/4wS+gnHS0owNoK02LTO9Ya0kjPQFw7T9/nZnMY34 4B5A== X-Gm-Message-State: APjAAAXSslnqyt9JGD1umn3cgGWMiyz/344svPTVW/+rts+ixuL4WSfZ 3+u0nTFa3EfYCNsaDMdj0eAOJw== X-Google-Smtp-Source: APXvYqwPlVls0+OdC+fpxFnkPZQI8Wl8gWPeJMPHbMb31mJ3YKa+yv+IWIroJp5SYgFfy9zPd2ZL7A== X-Received: by 2002:a81:7d8b:: with SMTP id y133mr25326480ywc.377.1568281827267; Thu, 12 Sep 2019 02:50:27 -0700 (PDT) Received: from localhost ([2600:380:9c1c:691f:cb4:3daf:a71a:adbc]) by smtp.gmail.com with ESMTPSA id t12sm4964438ywf.43.2019.09.12.02.50.25 (version=TLS1_2 cipher=ECDHE-RSA-AES128-GCM-SHA256 bits=128/128); Thu, 12 Sep 2019 02:50:26 -0700 (PDT) Date: Thu, 12 Sep 2019 05:50:23 -0400 From: Josef Bacik To: Anand Jain Cc: Josef Bacik , Eli V , linux-btrfs Subject: Re: [PATCH RFC v2 0/2] readmirror feature Message-ID: <20190912095021.htmpvvowdprc2jhv@MacBook-Pro-91.local> References: <20190826090438.7044-1-anand.jain@oracle.com> <20190911184229.gl7tu3igtuuepcvm@macbook-pro-91.dhcp.thefacebook.com> <20190911191656.mrmfyhvy3latjwid@macbook-pro-91.dhcp.thefacebook.com> <2f10bebf-bc63-fe9e-d7d3-06b3113bc95c@oracle.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <2f10bebf-bc63-fe9e-d7d3-06b3113bc95c@oracle.com> User-Agent: NeoMutt/20180716 Sender: linux-btrfs-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-btrfs@vger.kernel.org On Thu, Sep 12, 2019 at 03:41:42PM +0800, Anand Jain wrote: > > > Thanks for the comments. More below. > > On 12/9/19 3:16 AM, Josef Bacik wrote: > > On Wed, Sep 11, 2019 at 03:13:21PM -0400, Eli V wrote: > > > On Wed, Sep 11, 2019 at 2:46 PM Josef Bacik wrote: > > > > > > > > On Mon, Aug 26, 2019 at 05:04:36PM +0800, Anand Jain wrote: > > > > > Function call chain __btrfs_map_block()->find_live_mirror() uses > > > > > thread pid to determine the %mirror_num when the mirror_num=0. > > > > > > > > > > This patch introduces a framework so that we can add policies to determine > > > > > the %mirror_num. And also adds the devid as the readmirror policy. > > > > > > > > > > The new property is stored as an item in the device tree as show below. > > > > > (BTRFS_READMIRROR_OBJECTID, BTRFS_PERSISTENT_ITEM_KEY, devid) > > > > > > > > > > To be able to set and get this new property also introduces new ioctls > > > > > BTRFS_IOC_GET_READMIRROR and BTRFS_IOC_SET_READMIRROR. The ioctl argument > > > > > is defined as > > > > > struct btrfs_ioctl_readmirror_args { > > > > > __u64 type; /* RW */ > > > > > __u64 device_bitmap; /* RW */ > > > > > } > > > > > > > > > > An usage example as follows: > > > > > btrfs property set /btrfs readmirror devid:1,3 > > > > > btrfs property get /btrfs readmirror > > > > > readmirror devid:1 3 > > > > > btrfs property set /btrfs readmirror "" > > > > > btrfs property get /btrfs readmirror > > > > > readmirror default > > > > > > > > > > This patchset has been tested completely, however marked as RFC for the > > > > > following reasons and comments on them (or any other) are appreciated as > > > > > usual. > > > > > . The new objectid is defined as > > > > > #define BTRFS_READMIRROR_OBJECTID -1ULL > > > > > Need consent we are fine to use this value, and with this value it > > > > > shall be placed just before the DEV_STATS_OBJECTID item which is more > > > > > frequently used only during the device errors. > > > > > > > > > > . I am using a u64 bitmap to represent the devices id, so the max device > > > > > id that we could represent is 63, its a kind of limitation which should > > > > > be addressed before integration, I wonder if there is any suggestion? > > > > > Kindly note that, multiple ioctls with each time representing a set of > > > > > device(s) is not a choice because we need to make sure the readmirror > > > > > changes happens in a commit transaction. > > > > > > > > > > v1->RFC v2: > > > > > . Property is stored as a dev-tree item instead of root inode extended > > > > > attribute. > > > > > . Rename BTRFS_DEV_STATE_READ_OPRIMIZED to BTRFS_DEV_STATE_READ_PREFERRED. > > > > > . Changed format specifier from devid1,2,3.. to devid:1,2,3.. > > > > > > > > > > RFC->v1: > > > > > Drops pid as one of the readmirror policy choices and as usual remains > > > > > as default. And when the devid is reset the readmirror policy falls back > > > > > to pid. > > > > > Drops the mount -o readmirror idea, it can be added at a later point of > > > > > time. > > > > > Property now accepts more than 1 devid as readmirror device. As shown > > > > > in the example above. > > > > > > > > > > > > > This is a lot of infrastructure > > Ok. Any idea on a better implementation? > How about extended attribute approach? v1 patches proposed > it, but it abused the extended attribute as commented here [1] > and v2 got changed to an item-key. > > [1] > https://lore.kernel.org/linux-btrfs/be68e6ea-00bc-b750-25e1-9c584b99308f@gmx.com/ > That's a NAK on the prop interface. This is a fs wide policy, not a directory/inode policy. > > > > > to just change which mirror we read to based on > > > > some arbitrary user policy. I assume this is to solve the case where you have > > > > slow and fast disks, so you can always read from the fast disk? And then it's > > > > only used in RAID1, so the very narrow usecase of having a RAID1 setup with a > > > > SSD and a normal disk? I'm not seeing a point to this much code for one > > > > particular obscure setup. Thanks, > > > > > > > > Josef > > > > > > Not commenting on the code itself, but as a user I see this SSD RAID1 > > > acceleration as a future much have feature. It's only obscure at the > > > moment because we don't have code to take advantage of it. But on > > > large btrfs filesystems with hundreds of GB of metadata, like I have > > > for backups, the usability of the filesystem is dramatically improved > > > having the metadata on an SSD( though currently only half of the time > > > due to the even/odd pid distribution.) > > > > But that's different from a mirror. 100% it would be nice to say "put my > > metadata on the ssd, data elsewhere". That's not what this patch is about, this > > patch is specifically about changing which drive we choose in a mirrored setup, > > which is super unlikely to mirror a SSD with a slow drive, cause it's just going > > to be slow no matter what. Sure we could make it so reads always go to the SSD, > > but we can accomplish that by just adding a check for nonrotational in the code, > > and then we don't have to encode all this nonsense in the file system. Thanks, > > I wrote about the readmirror policy framework here[2], > I forgot to link it here, sorry about that, my mistake. > > [2] > > https://lore.kernel.org/linux-btrfs/1552989624-29577-1-git-send-email-anand.jain@oracle.com/ > > Readmirror policy is for raid1, raid10 and future N way mirror. > Yes for now its only for raid1. > > Here the idea is to create a framework so that readmirror policy > can be configured as needed. And nonrotational can be one such policy. > > The example of hard-coded nonrotational policy does not work in case > of ssd and a remote iscsi ssd, OR in case of local ssd and a NVME block > device, as all these are still nonrotational devices. So hard-coded > policy is not a good idea. If we have to hardcode then there is Q-depth > based readmirror routing is better (patch in the ML), but that is > not good enough, because some configs wants it based on the disk-LBA > so that SAN storage target cache is balanced and not duplicated. > So in short it must be a configurable policy. > Again, if you are mixing disk types you likely always want non-rotational, but still mixing different speed devices in a mirror setup is just asking for weird latency problems. I don't think solving this use case is necessary. If you mix ssd + network device in a serious production setup then you probably should be fired cause you don't know what you are doing. Having the generic "nonrotational gets priority" is going to cover 99% of the actual use cases that make sense. The SAN usecase I can sort of see, but again I don't feel like it's a problem we need to solve with on-disk format. Add a priority to sysfs so you can change it with udev or something on the fly. Thanks, Josef