Server Guide part 2: Affordable and Manageable Storage
by Johan De Gelas on October 18, 2006 6:15 PM EST- Posted in
- IT Computing
Enterprise Disks: all about SCSI
There are currently only two kinds of hard disks: those which work with SCSI commands and those which work with (S)ATA commands. However, those SCSI commands can be sent over three disk interfaces:
Will SATA kill off SCSI?
One look at the price of a typical "enterprise disk" -- whether it be a SCSI, FC or SAS disk -- will tell you that you have to pay at least 5 times and up to 10 times more per GB. Look at the specification sheets and you will see that the advantage you get for paying this enormous price premium seems to be only a 2 to 2.5 times lower access time (seek + latency) and a maximum transfer rate that is perhaps 20 to 50% better.
In the past, the enormous price difference between the disks which use ATA commands and the disks which use SCSI commands could easily be explained by the fact that a PATA disk would simply choke when you sent a lot of random concurrent requests. As quite a few of reviews here at AnandTech have shown, thanks to Native Command Queuing the current SATA drives handle enterprise workloads quite well. The number of concurrent I/O operations per second is easily increased by 50% thanks to NCQ. So while PATA disks were simply pathetically slow, the current SATA disks are - in the worst case - about half as fast as their SCSI counterparts when it comes to typical I/O intensive file serving.
There is more. The few roadblocks that kept SATA out of the enterprise world have also been cleared. One of the biggest problems was the point to point nature of SATA: each disks needs its own cable to the controller. This results in a lot of cable clutter which made SATA-I undesirable for enterprise servers or storage rack enclosures.
This roadblock can be removed in two ways. The first way is to use a backplane with SATA port multipliers. A port multiplier can be compared to a switch. One Host to SATA connection is multiplexed to multiple SATA connectors. At most 15 disks can make us of one SATA point to point connection. In reality, port multipliers connect 4 to 8 disks per port. As the best SATA disks are only able to sustain about 60 to 80 MB/s in the outer zones, a four disk port multiplier make sense even for streaming applications. For more random applications, even 8 or 15 disks on one 300 MB/s SATA connection would not result in a bottleneck.
Port multipliers are mostly used on the back panel of a server or the backplane of a storage rack. The second way is to use your SATA disks in a SAS enclosure. We will discuss this later.
There are currently only two kinds of hard disks: those which work with SCSI commands and those which work with (S)ATA commands. However, those SCSI commands can be sent over three disk interfaces:
- SCSI-320 or 16 bit Parallel SCSI
- SAS or Serial Attached SCSI
- FC or Fibre Channel
Will SATA kill off SCSI?
One look at the price of a typical "enterprise disk" -- whether it be a SCSI, FC or SAS disk -- will tell you that you have to pay at least 5 times and up to 10 times more per GB. Look at the specification sheets and you will see that the advantage you get for paying this enormous price premium seems to be only a 2 to 2.5 times lower access time (seek + latency) and a maximum transfer rate that is perhaps 20 to 50% better.
In the past, the enormous price difference between the disks which use ATA commands and the disks which use SCSI commands could easily be explained by the fact that a PATA disk would simply choke when you sent a lot of random concurrent requests. As quite a few of reviews here at AnandTech have shown, thanks to Native Command Queuing the current SATA drives handle enterprise workloads quite well. The number of concurrent I/O operations per second is easily increased by 50% thanks to NCQ. So while PATA disks were simply pathetically slow, the current SATA disks are - in the worst case - about half as fast as their SCSI counterparts when it comes to typical I/O intensive file serving.
There is more. The few roadblocks that kept SATA out of the enterprise world have also been cleared. One of the biggest problems was the point to point nature of SATA: each disks needs its own cable to the controller. This results in a lot of cable clutter which made SATA-I undesirable for enterprise servers or storage rack enclosures.
This roadblock can be removed in two ways. The first way is to use a backplane with SATA port multipliers. A port multiplier can be compared to a switch. One Host to SATA connection is multiplexed to multiple SATA connectors. At most 15 disks can make us of one SATA point to point connection. In reality, port multipliers connect 4 to 8 disks per port. As the best SATA disks are only able to sustain about 60 to 80 MB/s in the outer zones, a four disk port multiplier make sense even for streaming applications. For more random applications, even 8 or 15 disks on one 300 MB/s SATA connection would not result in a bottleneck.
Port multipliers are mostly used on the back panel of a server or the backplane of a storage rack. The second way is to use your SATA disks in a SAS enclosure. We will discuss this later.
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slashbinslashbash - Thursday, October 19, 2006 - link
Sounds great, thanks. If possible it'd be great to see full schematics of the setup, pics of everything, etc. This is obviously outside the realm of your "everyday PC" stuff where we all know what's going on. I administer 6 servers at a colo facility and our servers (like 90% of the other servers that I see) are basically PC hardware stuck in a rackmount box (and a lot of the small-shop webhosting companies at the colo facility use plain towers! In the rack across from ours, there are 4 Shuttle XPC's! Unbelievable!).We use workstation motherboards with ECC RAM, Raptor drives, etc. but still it's basically just a PC. These external enclosures, SAS, etc. are a whole new realm. I know that it'd be better than the ad-hoc storage situation we have now, but I'm kind of scared because I don't know how it works and I don't know how much it would cost. So now I know more about how it works, but the cost is still scary. ;)
I guess the last thing I'd want to know is the OS support situation. Linux support is obviously crucial.