From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from plane.gmane.org ([80.91.229.3]:58243 "EHLO plane.gmane.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1750732Ab3EUCW4 (ORCPT ); Mon, 20 May 2013 22:22:56 -0400 Received: from list by plane.gmane.org with local (Exim 4.69) (envelope-from ) id 1UecE1-0001T3-9n for linux-btrfs@vger.kernel.org; Tue, 21 May 2013 04:22:53 +0200 Received: from ip68-231-22-224.ph.ph.cox.net ([68.231.22.224]) by main.gmane.org with esmtp (Gmexim 0.1 (Debian)) id 1AlnuQ-0007hv-00 for ; Tue, 21 May 2013 04:22:53 +0200 Received: from 1i5t5.duncan by ip68-231-22-224.ph.ph.cox.net with local (Gmexim 0.1 (Debian)) id 1AlnuQ-0007hv-00 for ; Tue, 21 May 2013 04:22:53 +0200 To: linux-btrfs@vger.kernel.org From: Duncan <1i5t5.duncan@cox.net> Subject: Re: [RFC PATCH v1 0/5] BTRFS hot relocation support Date: Tue, 21 May 2013 02:22:34 +0000 (UTC) Message-ID: References: <1369062687-23544-1-git-send-email-zwu.kernel@gmail.com> Mime-Version: 1.0 Content-Type: text/plain; charset=UTF-8 Cc: linux-kernel@vger.kernel.org Sender: linux-btrfs-owner@vger.kernel.org List-ID: zwu.kernel posted on Mon, 20 May 2013 23:11:22 +0800 as excerpted: > The patchset is trying to introduce hot relocation support > for BTRFS. In hybrid storage environment, when the data in rotating disk > get hot, it can be relocated to nonrotating disk by BTRFS hot relocation > support automatically; also, if nonrotating disk ratio exceed its upper > threshold, the data which get cold can be looked up and relocated to > rotating disk to make more space in nonrotating disk at first, and then > the data which get hot will be relocated to nonrotating disk > automatically. One advantage of a filesystem implementation, as opposed to bcache or dmcache, is arguably a corner-case, but it's /my/ corner-case, so... I run an intr*-less (I guess technically, empty initramfs) monolithic- kernel boot, using the kernel commandline root= and (formerly) md= and related logic to choose/assemble/mount root directly from the kernel command line via bootloader (grub2). Thus, any user-space-required-to- mount-root is out, since I don't have an initr* and thus no early userspace. That means both lvm2 and dmcache (AFAIK) are out. I'm not sure about bcache, but it has other negatives, particularly against btrfs- raid-1 and I'd guess md/raid-1 as well. Much like md before it, btrfs, while normally requiring the user-space- required device-scan to properly handle multiple devices, has kernel- command-line options that allow direct kernel multi-device assembly without the help of early-userspace/initr*. So in-btrfs hot-relocation support building on the existing kernel- command-line multi-device assembly options would definitely be welcomed by all us no-initr* folks looking at SSDs but not able to afford them for /everything/ just yet. =:^) (That said, even if accepted, your solution's a bit late for my own current needs, but there's surely going to be others hitting the same issue in a few kernel cycles when your patches could be mainline btrfs, and having the option at my next upgrade cycle say a couple years out would be very nice, indeed. =:^) -- Duncan - List replies preferred. No HTML msgs. "Every nonfree program has a lord, a master -- and if you use the program, he is your master." Richard Stallman