Friday, March 30, 2012

NAS vs SAN

SAS vs SATA

Here are the high-level differences between SAS and SATA disk drives:

Capacity:
  • SATA disk drives are the largest on the market. The largest SATA drives available with widespread distribution today are 1.5TB-2TB.
  • SAS disk drives are typically smaller than SATA. The largest SAS drives available with widespread distribution today are 450GB.
  • So, for capacity, a SATA disk drive is 3X-4x as dense for capacity than SAS.
  • A good way to quantify capacity comparison is $/GB. SATA will have best $/GB.
Performance:
  • SATA disk drives spin at 7.2k RPMs. Average seek time on SATA is 9.5msec. Raw Disk IOPS (IOs per second) are 106.
  • SAS disk drives spin at 15k RPMs. Average seek time on SATA is 3.5msec. Raw Disk IOPS (IOs per second) are 294.
  • So, for performance, a SAS hard drive is nearly 3X as fast as SATA.
  • A good way to quantify performance comparison is $/IOP. SAS will have best $/IOP.
Reliability: there are two reliability measures – MTBF and BER.
  • MTBF is mean time between failures. MTBF is a statistical measure of drive reliability.
  • BER is Bit Error Rate. BER is a measure of read error rates for disk drives.
  • SATA drives have a MTBF of 1.2 million hours. SAS drives have a MTBF of 1.6 million hours. SAS drives are more reliable than SATA when looking at MTBF.
  • SATA drives have a BER of 1 read error in 10^15 bits read. SAS drives have a BER of 1 read error in 10^16 bits read. SAS drives are 10x more reliable for read errors. Keep in mind a read error is data loss without other mechanisms (RAID or Network RAID) in place to recover the data.