From owner-freebsd-questions@FreeBSD.ORG Thu May 21 15:38:16 2015 Return-Path: Delivered-To: freebsd-questions@freebsd.org Received: from mx1.freebsd.org (mx1.freebsd.org [IPv6:2001:1900:2254:206a::19:1]) (using TLSv1.2 with cipher AECDH-AES256-SHA (256/256 bits)) (No client certificate requested) by hub.freebsd.org (Postfix) with ESMTPS id 30070AA for ; Thu, 21 May 2015 15:38:16 +0000 (UTC) Received: from mail-lb0-x230.google.com (mail-lb0-x230.google.com [IPv6:2a00:1450:4010:c04::230]) (using TLSv1.2 with cipher ECDHE-RSA-AES128-GCM-SHA256 (128/128 bits)) (Client CN "smtp.gmail.com", Issuer "Google Internet Authority G2" (verified OK)) by mx1.freebsd.org (Postfix) with ESMTPS id B9B051D7A for ; Thu, 21 May 2015 15:38:15 +0000 (UTC) Received: by lbbuc2 with SMTP id uc2so21031856lbb.2 for ; Thu, 21 May 2015 08:38:13 -0700 (PDT) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=gmail.com; s=20120113; h=mime-version:date:message-id:subject:from:to:content-type; bh=w81hACDP6WBt+NrbJhbZbHA09lCuRjG4Tytygsl0/54=; b=vHdFGoJFw/Yyk0TzaLPrBhU3A5clfKqJlHsDNVfVOc9Od563xGQyXh/evt6Vlqfnsa 1FUm/H201WwpkaRsLAJOsWkVfUgJwATm97OGjO7YkVghK/x3bZ8uaASz9yAnGmU3CiHE Aax8iJjuKpAKO0K/qbrL3uoRL4fR7J5hJAmbmpr41z2HHkaQ/1VRo0gnpPZOQBRaCZGn beynq+5TfSeMXeCVh+GDnkIB4sxP4ciKewK7WMLtYyBQPPdN+7qIs43BmqhtofK7DHGk 8voD8HfrNWUwiWpC8mk0HbQHJZ7QjNzeKj7d0525gWpIrtF8VnnOv7k6Q67hbc5VXAZV 3NwA== MIME-Version: 1.0 X-Received: by 10.112.199.1 with SMTP id jg1mr2832936lbc.59.1432222693642; Thu, 21 May 2015 08:38:13 -0700 (PDT) Received: by 10.25.42.146 with HTTP; Thu, 21 May 2015 08:38:13 -0700 (PDT) Date: Thu, 21 May 2015 08:38:13 -0700 Message-ID: Subject: SLOG and SSDs: are "super" capacitors really needed? From: Chris Stankevitz To: freebsd-questions Content-Type: text/plain; charset=UTF-8 X-BeenThere: freebsd-questions@freebsd.org X-Mailman-Version: 2.1.20 Precedence: list List-Id: User questions List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , X-List-Received-Date: Thu, 21 May 2015 15:38:16 -0000 When sync data is written to the ZIL (or in my case to a SLOG), ZFS waits for the write to be "completed" before continuing. Once the write has "completed", the sync data is considered written, even if it has not yet made it to the real storage devices. Written data has "completed" when the ZIL device (SLOG) reports that the data has been written. Question: do SSD drives report the write has "completed" only after the data has been burned into non-volatile storage? If so, then why do people say a good SLOG SSD has "super capacitors" that allow the drive to continue functioning for a short time after a power failure? It seems to me that there are two scenarios, none of which need super capacitors: 1. A transaction is completely written to the SLOG, but not the storage devices, and the power goes out. No problem, data will write to storage when the pool is imported. 2. A transaction is partially written to the SLOG, but not the storage devices, and the power goes out. No problem, the transaction will be lost and the pool will be imported with the previously committed data/transaction. I don't see a scenario where a power-outage causes a "corrupted transaction" to be posted. Now if an SSD reports data "written" before it makes it to non-volatile storage, then that is another story... but I cannot imagine a HDD manufacturer advertising data written that is not actually written (or guaranteed to be written even in the face of a power outage). Thank you, Chris