From owner-freebsd-fs@freebsd.org Sat Aug 8 11:41:14 2015 Return-Path: Delivered-To: freebsd-fs@mailman.ysv.freebsd.org Received: from mx1.freebsd.org (mx1.freebsd.org [IPv6:2001:1900:2254:206a::19:1]) by mailman.ysv.freebsd.org (Postfix) with ESMTP id 288099B6310 for ; Sat, 8 Aug 2015 11:41:14 +0000 (UTC) (envelope-from kostikbel@gmail.com) Received: from mailman.ysv.freebsd.org (mailman.ysv.freebsd.org [IPv6:2001:1900:2254:206a::50:5]) by mx1.freebsd.org (Postfix) with ESMTP id 0C4FA14EB for ; Sat, 8 Aug 2015 11:41:14 +0000 (UTC) (envelope-from kostikbel@gmail.com) Received: by mailman.ysv.freebsd.org (Postfix) id 0B8969B630F; Sat, 8 Aug 2015 11:41:14 +0000 (UTC) Delivered-To: fs@mailman.ysv.freebsd.org Received: from mx1.freebsd.org (mx1.freebsd.org [IPv6:2001:1900:2254:206a::19:1]) by mailman.ysv.freebsd.org (Postfix) with ESMTP id 0A2A69B630E for ; Sat, 8 Aug 2015 11:41:14 +0000 (UTC) (envelope-from kostikbel@gmail.com) Received: from kib.kiev.ua (kib.kiev.ua [IPv6:2001:470:d5e7:1::1]) (using TLSv1 with cipher DHE-RSA-CAMELLIA256-SHA (256/256 bits)) (Client did not present a certificate) by mx1.freebsd.org (Postfix) with ESMTPS id 8738A14E9 for ; Sat, 8 Aug 2015 11:41:13 +0000 (UTC) (envelope-from kostikbel@gmail.com) Received: from tom.home (kostik@localhost [127.0.0.1]) by kib.kiev.ua (8.15.2/8.15.2) with ESMTPS id t78Bf7Lw023254 (version=TLSv1 cipher=DHE-RSA-CAMELLIA256-SHA bits=256 verify=NO); Sat, 8 Aug 2015 14:41:08 +0300 (EEST) (envelope-from kostikbel@gmail.com) DKIM-Filter: OpenDKIM Filter v2.10.3 kib.kiev.ua t78Bf7Lw023254 Received: (from kostik@localhost) by tom.home (8.15.2/8.15.2/Submit) id t78Bf7jB023253; Sat, 8 Aug 2015 14:41:07 +0300 (EEST) (envelope-from kostikbel@gmail.com) X-Authentication-Warning: tom.home: kostik set sender to kostikbel@gmail.com using -f Date: Sat, 8 Aug 2015 14:41:07 +0300 From: Konstantin Belousov To: Willem Jan Withagen Cc: fs@freebsd.org Subject: Re: Using SSDs as swap Message-ID: <20150808114107.GD2072@kib.kiev.ua> References: <55C5D48E.6010605@digiware.nl> <20150808102900.GA2072@kib.kiev.ua> <20150808103810.GB2072@kib.kiev.ua> <55C5E697.4080102@digiware.nl> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <55C5E697.4080102@digiware.nl> User-Agent: Mutt/1.5.23 (2014-03-12) X-Spam-Status: No, score=-2.0 required=5.0 tests=ALL_TRUSTED,BAYES_00, DKIM_ADSP_CUSTOM_MED,FREEMAIL_FROM,NML_ADSP_CUSTOM_MED autolearn=no autolearn_force=no version=3.4.1 X-Spam-Checker-Version: SpamAssassin 3.4.1 (2015-04-28) on tom.home X-BeenThere: freebsd-fs@freebsd.org X-Mailman-Version: 2.1.20 Precedence: list List-Id: Filesystems List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , X-List-Received-Date: Sat, 08 Aug 2015 11:41:14 -0000 On Sat, Aug 08, 2015 at 01:23:03PM +0200, Willem Jan Withagen wrote: > On 8-8-2015 12:38, Konstantin Belousov wrote: > > On Sat, Aug 08, 2015 at 01:29:00PM +0300, Konstantin Belousov wrote: > >> On Sat, Aug 08, 2015 at 12:06:06PM +0200, Willem Jan Withagen wrote: > >>> one of the following commits just passed with this in the log, and it > >>> triggered again a question I've been having for some time again already. > >>> > >>> ---- > >>> Log: > >>> Enable BIO_DELETE passthru in GELI, so TRIM/UNMAP can work as expected > >>> when > >>> GELI is used on a SSD or inside virtual machine, so that guest can tell > >>> host that it is no longer using some of the storage. > >>> ----- > >>> > >>> In ZFS I slice my SSD's into log and caches, but on a a server with > >>> little memory (which can't be grown) I use a partion on each ssd as swap > >>> as well. So swappinging does not have to seek, and has faster loading > >>> time. To allocate a few GB on aan SSD to swap is not really all that > >>> painfull, given current sizes, but the speed difference with regular > >>> spindels is impressive. > >>> > >>> But the questions are: > >>> 1) Does the swap driver understand that backing-store needs a TRIM? > >> No. > >> > >>> 1a) if not would it be useful, and what would it take to implement? > >> One good thing is that it is simply the question of coding: the VM > >> already has a place where it informs the swap pager that the page copy > >> in swap is no longer needed. this is the vm_pager_page_unswapped() call > >> and swap pager method swap_pager_unswapped(). swp_pager_meta_ctl() would > >> need to issue BIO_DELETE to the backing storage. > >> > >> On the other hand, note that this would increase the amount of work > >> performed, even for the swap volumes located on the rotating media, > >> which is more typical and reasonable setup. > >> > >> I think an implementation and a knob to turn it off, or configure per > >> swap partition, would be reasonable. > > > > One additional thing: while BIO_DELETE is in progress, the swap block > > cannot be marked free, since otherwise we could write other page and > > get it obliterated with the TRIM. This can be done async, but the > > consequence is that swap space would be released and usable some time > > after the page-in. This will affect loads which are close to OOM. > > Sort of makes sense to me... > > I take it that BIO_DELETE fires and returns before TRIM is completed? > But then the SSD accepts writes to a TRIMmed block, but then mixes this > up? Possibly deleting a write to a to be trimmed block? This sort of > strikes me as odd, but then I do not know the full intricate details of > TRIM on SSD > > Would it be possible to be notified that a TRIM has completed, only then > to actually free the swap sectors? This is exactly what I wrote above. > And then perhaps the swap bookkeeping does not yet accommodate for a > possible extra state? It does not need to. The in-flight BIO_DELETE remembers the intermediate state, the swap block should be freed only after the storage reported the BIO_DELETE as finished. It is exactly the same as UFS handles trimming of the free blocks, the bitmap of the used/freed blocks is only updated after the BIO_DELETE is finished, not when the inode drops reference to the block. > > Speaking about blocks.... Does Swap take into account that disks could > be of a sectorsize other than 512 bytes. I would guess so, since we > could have a 4K disk as swap disk, and doing read-modify-write for swap > is sure going to kill performance. swap performs i/o in the page-sized chunks at least, which are min 4k on all supported platforms (even on arms, where we do not support smaller pages AFAIK).