November 9, 200916 yr Where's the bottleneck when using unraid? is it disk speed? motherboard bandwidth? controller bandwidth? processor? Would I get better write performance if I switch to some pci express SATAII controllers and fast drives? or would I benefit more from a faster processor? (running a 3.2ghz p4 w/ hyperthreading 2gb of ram)
November 9, 200916 yr Drive speed and bus bandwidth (controller) are the top factors. Most motherboard built-in controllers have optimal bandwidth so converting from them to PCI Express wont be worth it. However, switching from PCI to PCI Express could be worth it, depending on the number of drives connected to the controller. IIRC, PCI has a limitation of 133 MB/s and a PCI Express single lane (1x) is 250 MB/s.
November 16, 200916 yr When writing to the array, and not using a "cache" drive, you will always be limited by the rotational speed of the disks involved. (parity AND data disk) See here: http://lime-technology.com/forum/index.php?topic=4338.msg38768#msg38768 Joe L.
November 16, 200916 yr Author When writing to the array, the rotational speed of the disks involved (both parity and the data disk) will usually be the limiting factor. Each block written must first be read from the drives involved and then THE PLATTER MUST SPIN AROUND, AT LEAST ONCE, TO GET THE DISK HEAD BACK TO THE SAME SECTOR TO WRITE IT. Something about this doesn’t make sense. How can it read something from the disk before it is written? When I copy some data to the server, does it write to the disk, then re-read that data from the disk in the unraid server, calculate the parity, and then write to the parity disk?
November 16, 200916 yr When writing to the array, the rotational speed of the disks involved (both parity and the data disk) will usually be the limiting factor. Each block written must first be read from the drives involved and then THE PLATTER MUST SPIN AROUND, AT LEAST ONCE, TO GET THE DISK HEAD BACK TO THE SAME SECTOR TO WRITE IT. Something about this doesn’t make sense. How can it read something from the disk before it is written? When I copy some data to the server, does it write to the disk, then re-read that data from the disk in the unraid server, calculate the parity, and then write to the parity disk? No, it does not re-read the data just written, but it must first read the existing contents of both the data block being over-written, and the existing contents of the parity block to be written, before it can write to either. This post may help (need to think of it at the individual bit level) : http://lime-technology.com/forum/index.php?topic=4390.msg40666#msg40666 Joe L.
November 16, 200916 yr Author Now I see why the writing process is so slow thanks for the links, very informative.
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