Date: Wed, 09 Aug 2017 16:53:38 +0100 From: "Frank Leonhardt (m)" <frank2@fjl.co.uk> To: Josh Paetzel <josh@tcbug.org>, freebsd-hardware@freebsd.org Subject: Re: Do I need SAS drives?.. Message-ID: <30F28D62-B262-47C6-929D-58902CBF37A6@fjl.co.uk> In-Reply-To: <1502292592.2001426.1068163912.1191A246@webmail.messagingengine.com> References: <4DFBCE11-913A-4FC9-937D-463B4D49816C@aldan.algebra.com> <362B0950-A244-4C65-89C7-898EFC6A4A1F@fjl.co.uk> <1502292592.2001426.1068163912.1191A246@webmail.messagingengine.com>
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On 9 August 2017 16:29:52 BST, Josh Paetzel <josh@tcbug.org> wrote: > > >On Wed, Aug 9, 2017, at 09:55 AM, Frank Leonhardt (m) wrote: >> Simple answer is to use either. You're running FreeBSD with ZFS, >right? >> BSD will hot plug anything. I suspect 'hot plug' relates to Microsoft >> workaround hardware RAID. >> >> Hot plug enclosures will also let the host know a drive has been >pulled. >> Otherwise ZFS won't know whether it was pulled or is unresponsive due >to >> it being on fire or something. With 8 drives in your array you can >> probably figure this out yourself. >> >> SAS drives use SCSI commands, which are supposedly better than SATA >> commands. Electrically they are the same. SAS drives are more >expensive >> and tend to be higher spec mechanically, but not always so. >Incidentally, >> nearline SAS is a cheaper SATA drive that understands SAS protocol >and >> has dual ports. Marketing. >> >> Basically, if you really want speed at all costs go for SAS. If you >want >> best capacity for your money, go SATA. If in doubt, go for SATA. If >you >> don't know you need SAS for some reason, you probably don't. >> >> Regards, Frank. >> >> >> On 9 August 2017 15:27:37 BST, "Mikhail T." <mi+m@aldan.algebra.com> >> 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! >> >-- > >I have a different take on this. For starters SAS and SATA aren't >electrically compatible. There's a reason SAS drives are keyed so you >can't plug them in to a SATA controller. It keeps the magic smoke >inside the drive. SAS controllers can tunnel SATA (They confusingly >call this STP (Not Spanning Tree Protocol, but SATA Tunneling Protocol) > >It's imperfect but good enough for 8 drives. You really do not want to >put 60 SATA drives in a SAS JBOD) > >SAS can be a shared fabric, which means a group of drives are like a >room full of people having a conversation. If someone starts screaming >and spurting blood it can disrupt the conversations of everyone in the >room. Modern RAID controllers are pretty good at disconnecting drives >that are not working properly but not completely dead. Modern HBAs not >so much. If your controller is an HBA trying to keep a SAS fabric >stable with SATA drives can be more problematic than if you use SAS >drives...and as Frank pointed out nearline SAS drives are essentially >SATA drives with a SAS interface (and are typically under a $20 >premium) > >If performance was an issue we'd be talking about SSDs. While SAS >drives do have a performance advantage over SATA in >multiuser/multiapplication environments (they have a superior queuing >implementation) it's not worth considering when the real solution is >SSDs. > >My recommendation is if you have SAS expanders and an HBA use SAS >drives. If you have direct wired SAS or a RAID controller you can use >either SAS or SATA. If your application demands performance or >concurrency get a couple SSDs. They'll smoke anything any spinning >drive can do. There are differences, but not relevant to an 8 drive system IME. Electrically SAS works at a higher voltage on the differential pair, which means the cables can be a lot longer. Most (all?) SAS Expanders can handle STP so talk to SATA drives, but in an eight-way config I doubt a SAS expander comes in to it - they're not cheap! Incidentally, SATA allows for expanders now. Okay, SAS has tagged command queueing, but SATA has native command queuing. Incidentally, the slightly different notched drive connector is simply to stop you plugging a SAS drive on to a SATA HBA, because it wouldn't know how to talk to it. It won't go bang if you do it by mistake. OTOH a SAS HBA can talk both, so has a notch to match the raised bit on SATA. Could go on about drives for ever (and have done in the past) but this is just an array of eight drives. Have you thought about Fibre channel :-) Regards, Frank. -- Sent from my Android device with K-9 Mail. Please excuse my brevity.
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