On Aug 22, 2010, at 1:16 PM, ah...clem wrote: > > > the difference between HD speed and interface speed has been > discussed on these lists many times. as eric correctly points out, > the SATA-II interface is capable of handling 3 GB/s. > > but, new HDs themselves currently have sustained read/write speeds > of around 120 MB/s or above. i have tested several using HDST that > were in the 120 MB/s range (seagate and maxtor). i have not > purchased the latest or most expensive HDs, so i assume that there > are probably others out there that may be a bit faster. i have also > tested PATA drives that were in the 100 MB/s range (seagate). this > number has increased steadily over the past 20 years from less than > 5 MB/s to the current level, and presumably will continue to increase. > the best SSDs currently advertise read/write speeds of 275/250 MB/s. > however, SSDs have a more limited number of read/write cycles > before they fail compared to a traditional spinning platter HD, so i > would NOT recommend getting an SSD to use as the boot drive. it > will wear out in a few years of normal use. in any case, for the > SATA-II interface to actually get to 3 GB/s would require two dozen > or more of the latest HDs in a RAID, so in practice, it is > unachievable. > in reality, a new PATA drive is likely just as fast as a SATA-II > drive, > if it's the only HD on the bus. Drive speeds are somewhat misleading. Often when you get a result from a benchmark, it's the result of the host talking to the controller about a relatively small file towards the edge of the disk where the data density is greatest and the speed is the highest. I deal with the latest and greatest disks every day building machines for people in high-performance situations, and so far the fastest disk I've tested is the newest generation of Seagate. The highest sustained data rate I saw in real-world situations was about 95MB/s. SSD's will likely be faster, but for mechanical drives, the speed is still increasing. Just 3-4 years ago, 60MB/s was considered blistering fast for a sustained data rate.
IDE/PATA drives are doing good to punch past about 60MB/s due to the overhead in their interface. SATA by it's nature has a lot less wasted time due to the fact that it's data is serial instead of parallel. On a parallel bus, the controller uses up a lot of time making sure that all data is sent and received at the same time. Serial doesn't care....it just sends everything out in order. This alone makes the SATA faster and less latent than the older PATA/IDE disks and controllers. While the drive is internally identical to the SATAII drives, it's the interface that slows them down. PATA disks are starting to be phased out by a lot of manufacturers, and I personally say GOOD RIDDANCE! SATA makes so many improvements in so many ways. >> > > and NO, it is not a "PCI thing." if you bother to actually think > about it, it should be obvious to anyone who's mastered sixth-grade > arithmetic. multiply the bus speed (MHz = million cycles/s) times > the > bus width (bits/cycle) and divide by 8 (bits/byte) and you will find > that > the PCI bus in an old Mac 9600 with a bus speed of 50 MHz is still > way faster than the latest HDs. > (50,000,000 cycles/s) x (32 bits/cycle) x (1 byte/8 bits) = > 200,000,000 bytes/s = 200 MB/s. in practice it will be a bit less, > but > still faster than any one single HD. You're leaving out an important note about the PCI bus. The speed of the PCI bus is the TOTAL capacity of the bus itself. Unlike PCI-Express, each slot does not have it's own data rate. It shares the total data allowance of the PCI bus itself. On the 32 bit PCI Macs the PCI bus speed is 33 Mhz with a width of 32 bits. It gives a maximum capacity of the bus as 133MB/s (hence why ATA/133 caps out there). On the 64 bit Macs, such as the G4's and G5's with PCI slots, the bus is still 33 Mhz, but it's 64 bits wide for a maximum throughput of 266MB/s. However the 32 bit front half of the slot is still limited to only 133 MB/s. Keep in mind that everything on a PCI bus has to share interrupts, wait states, and data throughput. So although you can have an ATA/133 drive, your performance is doing good if you even achieve half of that. It's because of all this sharing and latency that the PCIe architecture came about. It's better in more ways that I care to count! If you only have a single drive on a PCI bus, you will see about 60-65MB/s as your peak speed achievable on any single disk. If you have 2 disks and have them copy data from one to the other on the same PCI bus, you can half that due to the limitations of the bus itself. -- You received this message because you are a member of G-Group, a group for those using G3, G4, and G5 desktop Macs - with a particular focus on Power Macs. The list FAQ is at http://lowendmac.com/lists/g-list.shtml and our netiquette guide is at http://www.lowendmac.com/lists/netiquette.shtml To post to this group, send email to g3-5-list@googlegroups.com For more options, visit this group at http://groups.google.com/group/g3-5-list
