On Tue, Feb 23, 2010 at 2:09 PM, Erik Trimble <erik.trim...@sun.com> wrote: > Al Hopper wrote: >> >> >> On Fri, Dec 4, 2009 at 1:06 AM, Erik Trimble <erik.trim...@sun.com >> <mailto:erik.trim...@sun.com>> wrote: >> >> Hey folks. >> >> I've looked around quite a bit, and I can't find something like this: >> >> I have a bunch of older systems which use Ultra320 SCA hot-swap >> connectors for their internal drives. (e.g. v20z and similar) >> >> I'd love to be able to use modern flash SSDs with these systems, >> but I have yet to find someone who makes anything that would fit >> the bill. >> >> I need either: >> >> (a) a SSD with an Ultra160/320 parallel interface (I can always >> find an interface adapter, so I'm not particular about whether >> it's a 68-pin or SCA) >> >> (b) a SAS or SATA to UltraSCSI adapter (preferably with a SCA >> interface) >> >> >> Hi Erik, >> >> One of the less well known facts about SCSI is that all SCSI commands are >> sent in legacy 8-bit mode. And it takes multiple SCSI commands to make a >> SCSI drive do something useful! Translation - it's s-l-o-w. Since one of >> the big upsides of an SSD is I/O Ops/Sec - get ready for a disappointment if >> you use SCSI based connection. Sure - after the drive has received the >> necessary commands it can move data blocks reasonably quickly - but the >> limit, in terms of an SSD will *definitely* be the rate at which commands >> can be received by the drive. This (8-bit command) design decision was >> responsible for SCSIs' long lasting upward compatibility - but it also >> turned into its achilles heel; that ultimately doomed SCSI to extinction. > > Really? I hadn't realized this was a problem with SSDs and SCSI. Exactly > how does this impact SSDs with a SAS connection, since that's still using > the SCSI command set, just over a serial link rather than a parallel one. > Or, am I missing something, and is SAS considerably different (protocol > wise) from traditional parallel SCSI?
The key difference here is that the SCSI protocol commands and other data are sent to/from the SAS drive at the same (high) speed over the serial link. And another point - SAS and SATA are full duplex. This is why parallel SCSI had to die - you simply can't send enough SCSI commands over a SCSI parallel link to keep a modern, mechanical, 7,200RPM drive busy - let alone an SSD. Think about that for a Second - the mechanical drive is probably going to max out at 400 to 500 I/O Ops/Sec. By way of contrast, todays SSDs will do 33,000+ I/O Ops/Sec (for a workload that is I/O Op/Sec intensive). And tomorrows SSDs are going to be much faster. > Given the enormous amount of legacy hardware out there that has parallel > SCSI drive bays (I mean, SAS is really only 2-3 years old in terms of server > hardware adoption), I am just flabbergasted that there's no parallel-SCSI > SSD around. Now you know why. There is simply no way to get around the parallel SCSI standard spec and the fact that *all* SCSI commands are sent 8-bits wide at the very slow (original) 8-bit rate. And if you do find a converter, you're going to be bitterly disappointed with the results - even with a low-end SSD. PS: I think if someone does build/sell a parallel SCSI -> SATA SSD converter board, they are going to get a very high percentage of them returned from angry customers telling them they get better performance from a USB key that they do with this piece of *...@$!$# converter. And it's going to be very difficult to explain to the customer why the convert board is so slow - and working perfectly. >> I understand exactly the problem you're solving - and you're not alone >> (got 4 V20Zs in a CoLo in Menlo Park CA that I maintain for Genunix.Org and >> I visit them less than once a year at great expense - both in terms of time >> and dollars)! IMHO any kind of a hardware "hack job" and a couple of 1.8" >> or 2.5" SATA SSDs, combined with an OpenSolaris plugin SATA controller, >> would be a better solution. But I don't like this solution any more than I'm >> sure you do! >> >> Please contact me offlist if you have any ideas and please let us know (on >> the list) how this works out for you. >> >> Regards, >> >> -- >> Al Hopper Logical Approach Inc,Plano,TX a...@logical-approach.com >> <mailto:a...@logical-approach.com> >> Voice: 972.379.2133 Timezone: US CDT >> OpenSolaris Governing Board (OGB) Member - Apr 2005 to Mar 2007 >> http://www.opensolaris.org/os/community/ogb/ogb_2005-2007/ > > I've got stacks of both v20z/v40z hardware, plus a whole raft of IBM xSeries > (/not/ System X) machines which really, really, really need an SSD for > improved I/O. At this point, I'd kill for a parallel SCSI -> SATA adapter > thingy; something that would plug into a SCA connector on one side, and a > SATA port on the other. I could at least hack together a mounting bracket > for something like that... > > > > > > -- > Erik Trimble > Java System Support > Mailstop: usca22-123 > Phone: x17195 > Santa Clara, CA > > -- Al Hopper Logical Approach Inc,Plano,TX a...@logical-approach.com Voice: 972.379.2133 Timezone: US CDT OpenSolaris Governing Board (OGB) Member - Apr 2005 to Mar 2007 http://www.opensolaris.org/os/community/ogb/ogb_2005-2007/ _______________________________________________ zfs-discuss mailing list zfs-discuss@opensolaris.org http://mail.opensolaris.org/mailman/listinfo/zfs-discuss
