From owner-freebsd-hardware@freebsd.org Wed Aug 9 15:59:16 2017 Return-Path: Delivered-To: freebsd-hardware@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 38B80DD017E; Wed, 9 Aug 2017 15:59:16 +0000 (UTC) (envelope-from asomers@gmail.com) Received: from mail-wr0-x22e.google.com (mail-wr0-x22e.google.com [IPv6:2a00:1450:400c:c0c::22e]) (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 C8A9476FBE; Wed, 9 Aug 2017 15:59:15 +0000 (UTC) (envelope-from asomers@gmail.com) Received: by mail-wr0-x22e.google.com with SMTP id y43so25542206wrd.3; Wed, 09 Aug 2017 08:59:15 -0700 (PDT) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=gmail.com; s=20161025; h=mime-version:sender:in-reply-to:references:from:date:message-id :subject:to:cc; bh=dz8yUgnrMCg4tFPB9xlOlypi9GdLXMQSmmy65spPk5U=; b=PpDWgjR1C/YJsOX0HYB+/2kxBc6SwWrb5vODdZlr1BvRf0MfJZnKtPDEILe0gzRobd 1XQ1UAOKS00jST8RuIo5gfRYVD0hHuRcbrPrCqc3xRYQXQY1MY4eT37WmaeybjvYKhlJ A+uuaRtVFVlL5EhhDRPNG0XRYq499Uc7s7itVuMtKOVOvVm+wiPzipSzBvA7NfXgTUpZ QpLp51VVz5V9zCxbdmoNdUCuQ/4Yp5IN6iwmEL0Knw2huZ36xkCdGsLfb8Jsg8fnDE8N ku6sqe3E29tnoXT22EOkQ7y5V+3jgYtmu/7u52Hc5lfQ3MHM+0/TR7qgqokaCpMSQ8Ak uMJg== X-Google-DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=1e100.net; s=20161025; h=x-gm-message-state:mime-version:sender:in-reply-to:references:from :date:message-id:subject:to:cc; bh=dz8yUgnrMCg4tFPB9xlOlypi9GdLXMQSmmy65spPk5U=; b=QcKy/mWo1dLMEaVTUKgFwKaaXfVQApu483BDV/CwUlUglVQTO0jYV4Ds3l2mTg7pkK L+BB3eiBsuC8k+vr+Y9uvDtkP6bYztRAybHvM5CeMrzGd1uPF3mkfNEipKobfskxvaSq vYD4xXbHJvYDfyOwRve+GmYgefowWV7zrI2Dwgu/TSj28PgSERH+SE/rbgJssnfdwTQl RZnpArHDD6pUEV8o8mv/zXUpgrMSfPLFML8qsxXxYQ2tBCvMBOFzsoVy+mQecFcI1tCW hpkFQr7o6tRRIifZ0HX7vx2nXM8GL47nrAhlag8jmein/smu3bQICNeKhdmw/6B/cuFK gAMQ== X-Gm-Message-State: AHYfb5joMo6aYf3exLx0RGu+7U71EJKzYnOGTFr2mGz+S/PRdf5HpT59 EnTdTFiPbdMub/WJLaZrmyzp/JvDlw== X-Received: by 10.223.174.209 with SMTP id y75mr5668371wrc.19.1502294354228; Wed, 09 Aug 2017 08:59:14 -0700 (PDT) MIME-Version: 1.0 Sender: asomers@gmail.com Received: by 10.28.208.3 with HTTP; Wed, 9 Aug 2017 08:59:13 -0700 (PDT) In-Reply-To: <4DFBCE11-913A-4FC9-937D-463B4D49816C@aldan.algebra.com> References: <4DFBCE11-913A-4FC9-937D-463B4D49816C@aldan.algebra.com> From: Alan Somers Date: Wed, 9 Aug 2017 09:59:13 -0600 X-Google-Sender-Auth: NuITEyLAB4lYAZ4VdGC7OeHpXPM Message-ID: Subject: Re: Do I need SAS drives?.. To: "Mikhail T." Cc: freebsd-hardware@freebsd.org, FreeBSD-scsi Content-Type: text/plain; charset="UTF-8" X-BeenThere: freebsd-hardware@freebsd.org X-Mailman-Version: 2.1.23 Precedence: list List-Id: General discussion of FreeBSD hardware List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , X-List-Received-Date: Wed, 09 Aug 2017 15:59:16 -0000 On Wed, Aug 9, 2017 at 8:27 AM, Mikhail T. wrote: > My server has 8 "hot-plug" slots, that can accept both SATA and SAS drives. SATA ones tend to be cheaper for the same features (like cache-sizes), what am I getting for the extra money spent on SAS? > > Asking specifically about the protocol differences... It would seem, for example, SATA can not be as easily hot-plugged, but with camcontrol(8) that should not be a problem, right? What else? Thank you! > -- > Sent from mobile device, please, pardon shorthand. Good question. First of all, hot-plugability has more to do with the controller than the protocol. Since you have a SAS controller, you should have no problem hot plugging SATA drives. But SAS drives still have a few advantages: 1) When a SATA drive goes into error recovery, it can lock up the bus indefinitely. This won't matter if your drives are directly connected to a SAS HBA. But if you have an expander with say, 4 SAS lanes going to the HBA, then a flaky SATA drive can reduce the bandwidth available to the good drives. 2) Even with NCQ, the SATA protocol is limited to queueing one or more write commands OR one or more read commands. You can't queue a mixture of reads and writes at the same time. SAS does not have that limitation. In this sense, SAS is theoretically more performant. However, I've never heard of anybody observing a performance problem that can be definitely blamed on this effect. 3) SAS drives have a lot of fancy features that you may not need or care about. For example, they often have features that are useful in multipath setups (dual ports, persistent reservations), their error reporting capabilities are more sophisticated than SMART, their self encrypting command set is more sophisticated, etc etc. 4) The SAS activity LED is the opposite of SATA's. With SATA, the LED is off for an idle drive or blinking for a busy drive. With SAS, it's on for an idle drive or blinking for a busy drive. This makes it easier to see at a glance how many SAS drives you have installed. I think some SATA drives have a way to change the LEDs behavior, though. 5) Desktop class SATA drives can spend an indefinite amount of time in error recovery mode. If your RAID stack doesn't timeout a command, that can cause your array to hang. But SAS drives and RAID class SATA drives will fail any command than spends too much time in error recovery mode. 6) But the most important difference isn't something you'll find on any datasheet or protocol manual. SAS drives are built to a higher standard of quality than SATA drives, and have accordingly lower failure rates. I'm guessing that you don't have an expander (since you only have 8 slots), so item 1 doesn't matter to you. I'll guess that item 3 doesn't matter either, or you wouldn't have asked this question. Item 5 can be dealt with simply by buying the higher end SATA drives. So item 6 is really the most important. If this system needs to have very high uptime and consistent bandwidth, or if it will be difficult to access for maintenance, then you probably want to use SAS drives. If not, then you can save some money by using SATA. Hope that helps. -Alan