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IDE and SATA

Overview

Enterprise class disk drives need to have higher RPMs and a lower storage capacity vs an entry-level disk drive.  Having said that this page attempts to cover the various disk types in use today. 
 
Direct Attached Storage
aka DAS.  traditional disk drives.  Max cable length is 30m. 
 
IDE
IDE is also known as ATA.  IDE can only handle one outstanding I/O request per channel. 
IDE generally runs on PCI-E (serial) slots or PCI-X (parallel) slots.  PCI stands for Peripheral Component Interconnect. 
Mostly ATA is a parallel interface (like SCSI). 
 
SATA
Latest incarnation of ATA/IDE.  It started to simplify ATA cabling.  SATA cables are thinner and are only 4 wires (serial communications) so they allow for better airflow. 
Serial Advanced Technology Attachment
For larger loads this isn't sufficient...SCSI and SATA with TCQ support mult I/O requests.  Tagged Command Q'ing allows for servicing out-of-order IO requests.  It reduces seek time by restructuring disk access requests. 
SATA tends to be cheaper than SCSI.  Ensure SATA supports NCQ (native command q'ing) which is similar to TCQ on SCSI.  SATA has higher capacity than SCSI or SAS...but at the cost of higher latency.  NCQ tries to abate this. 
SATA link speeds are 1.5Gbps or 3.0 Gbps. 
Most SATA drives do not honor the OS write cache settings.  This means critical databases should probably not be run on SATA drives.  
 
SCSI tends to be faster than SATA due to higher rotational speeds and lower latency and seek times. 
 
 
 
SAS (Serial Attached SCSI)
  • The drive connects directly to a SAS port unlike SCSI which is a bus. 
  • SAS backplanes can accomodate both SAS and SATA devices. 
  • Performs better than SCSI, has a smaller form factor
  • Compatible with SATA. 
  • Uses STP for communication (SATA Tunneled Protocol). 
  • Uses the SCSI command language
  • Has dual ports on the disk for high availability (like FC)
  • Link speeds are either 1.5 Gbps or 3.0 Gbps
    • maximum throughput of a 4 phy SAS wide port that has 3 Gbps links is 2400 MBps. 
 

SATA Vs. SAS

Feature SAS SATA
Command Queuing TCQ...queue depths up to 216, some are 64.   NCQ...queue depths <= 32
Error recovery and detection Uses the SCSI command is more robust SMART Proven to be in adequate. see Google  Paper
Duplex Full Duplex dual port per drive Half Duplex single port
Multi-path IO fully supported at drive level supported in SATA II via expanders
 
  • FC can be both the linking technology between physical disks, as well as the connecting technology in a "switched fabric" or a fiber loop. 
 
All About SCSI
  • traditionally combines storing and connecting functions
  • Most SANs using SCSI processes
  • Some SANs use FICON (fiber connections such as FC).
  • SCSI connectivity is between an initiator and a target.  See HBAs
  • It is a bus connecting technology (vs FibreChannel loops) that requires electrical terminators on both ends of the bus. 
  • A SCSI nexus is the relationship between an initiator and a target.  (this is really nothing special, just a term SAN guys use to confuse us). 
  • Uses RPCs
  • asynchronous.  As such the HBA is interrupt-heavy for the host OS...definitely ensure you have the latest approved HBA drivers and that they are fully supported by your SAN vendor. 
  • overlapped IOs provide parallelism. 
  • dual mode controllers are available which have both an initiator and a target role.  Good for VSANs. 
  • ports are 0-15.  But the port ID on an FC fabric network is a 24bit address. 
  • Each port has a WWN (worldwide name).  64bit hex similar to a MAC address.   WWN/WWPN
  • LU:  Logical Unit.  a SCSI component that receives and processes commands on behalf of a storage address space.  LUs allow for the scalability beyond the 15 device limit in parallel SCSI. 
  • A LUN is a logical unit or access point for exchanging commands beween initiators and targets.  LUNs
  • Parallel SCSI:  cable distances can be 1.5m to 25m. 
Solid State Disks
combine high I/O with low latency. 
 
 
MultiPathing is another networking technology. 
 
 

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