From owner-freebsd-questions@freebsd.org Thu Sep 10 15:00:34 2015 Return-Path: Delivered-To: freebsd-questions@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 24773A018CF for ; Thu, 10 Sep 2015 15:00:34 +0000 (UTC) (envelope-from brandon.wandersee@gmail.com) Received: from mail-ig0-f169.google.com (mail-ig0-f169.google.com [209.85.213.169]) (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 EB10A18AC for ; Thu, 10 Sep 2015 15:00:33 +0000 (UTC) (envelope-from brandon.wandersee@gmail.com) Received: by igcrk20 with SMTP id rk20so18499618igc.1 for ; Thu, 10 Sep 2015 08:00:32 -0700 (PDT) X-Google-DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=1e100.net; s=20130820; h=references:from:to:cc:subject:in-reply-to:date:message-id :mime-version:content-type; bh=sX1KYw/vbEMNTLWJPbCa5wGVIxwyCGO2ujAGvF+Of+g=; b=F6f9IxUnU+srqYGYIpidANTJeOuCgX6UxLGgMh9Ahk+M7x+iT1GlbMb7mNmRkbZCI/ cOPmyE5nBGBjTzgklWrUil+ldd3a3NlcxoiRjAlRG4qAwwpFhE7c2lZ+Z9epd7V2WF8a fVd30effCs/Rv1fyTclc651AW1aaGuO/iunAumGS3ew+rOnCJwzgDpFXM+hHESvE8mFN vkJiYq7fDGeFrBG3GCeapfmYyPXPrgeYrYrOscqUEZBHFHUmEFTeTGJyMY5xOlmMjsUn 1PHqf+MkNvSmWyLzhOltDwiyJy/VnAAON+TiYqpgipoE2GapVhc3oHkhu3oKeS3WL43q 0uNg== X-Received: by 10.50.66.197 with SMTP id h5mr6851770igt.82.1441897231908; Thu, 10 Sep 2015 08:00:31 -0700 (PDT) Received: from WorkBox.Home.gmail.com (174-30-208-115.mpls.qwest.net. [174.30.208.115]) by smtp.gmail.com with ESMTPSA id os9sm4384164igb.11.2015.09.10.08.00.30 (version=TLSv1.2 cipher=ECDHE-RSA-AES128-GCM-SHA256 bits=128/128); Thu, 10 Sep 2015 08:00:30 -0700 (PDT) References: <20150829220311.c7608be1.freebsd@edvax.de> <55E45973.2050103@sneakertech.com> <55E4865B.1000104@sneakertech.com> <20150831181135.7682a810@gumby.homeunix.com> From: Brandon J. Wandersee To: RW Cc: freebsd-questions@freebsd.org Subject: Re: Replacing Drive with SSD In-reply-to: <20150831181135.7682a810@gumby.homeunix.com> Date: Thu, 10 Sep 2015 10:00:29 -0500 Message-ID: <867fnywc8i.fsf@WorkBox.Home> MIME-Version: 1.0 Content-Type: text/plain 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, 10 Sep 2015 15:00:34 -0000 RW via freebsd-questions writes: > On Mon, 31 Aug 2015 12:52:43 -0400 > Quartz wrote: > >> > >> > It provides a pool of blocks that have not and will not be >> > written. >> >> Bbut how does the drive "know" that those blocks are not allocated by >> a partition somewhere and are safe to use as spares? It's not like >> the drive firmware reads into the partition table or anything. > > It's analogous to virtual memory. It's the unused physical blocks that > form the free pool. The point of leaving the unpartitioned space is > that it's a region of a logical address space with no physical blocks > attached. Just to expand on this a little more (since it confused me as well after I bought my first SSD): solid-state drives do not write data to contiguous blocks, and "partitioning" the drive does not place actual barriers at physical points on the disk as is done with an HDD. The operating system sees SSD and HDD paritions in the same way, but on an SSD partitions function more like filesystem quotas. Leaving blocks unpartitioned doesn't mean those blocks won't be written to; it just ensures that a certain amount of space (and so a certain number of blocks/cells) must always be free, forcing writes to be spread across different physical portions of the disk in a relatively even manner. It's my understanding that this over-provisioning isn't exactly necessary in most SSDs nowadays. It just serves as a stronger guarantee than entrusting everything to a drive's firmware. -- ================================================================= :: Brandon Wandersee :: :: brandon.wandersee@gmail.com :: ================================================================== 'A common mistake that people make when trying to design something completely foolproof is to underestimate the ingenuity of complete fools.' - Douglas Adams ==================================================================