Hi, Does mount point specification while partitioning (order in which I specify /, /boot, swap etc..) affect performance? I am not sure about the syntax, but I guess one can also specify address/block range while partitioning. Does it affect IO performance? Probably a stupid question, but just curious.. Any insights? Thanks, CS.
Carlos Santana wrote:> Hi, > > Does mount point specification while partitioning (order in which I > specify /, /boot, swap etc..) affect performance? I am not sure about > the syntax, but I guess one can also specify address/block range while > partitioning. Does it affect IO performance? Probably a stupid > question, but just curious.. Any insights?Not a stupid question at all. For ordinary disk drives the answer is yes, absolutely. The outer tracks of a disk are physically longer, and any but the most ancient of disk drives will pack more sectors into those tracks. Since the disk rotates at a constant RPM, more sectors per second pass under the head on the outer tracks. The ratio of data rates for the outermost vs. innermost tracks is typically 2:1 or a bit higher. Add to this the need for more and longer seeks for filesystems on the inner tracks (again, less data on each physical track), and the performance degrades even more. On most disks cylinder numbering starts at the outer tracks, but I have heard of disks that number their cylinders in the opposite direction -- never actually seen one, though. -- Bob Nichols "NOSPAM" is really part of my email address. Do NOT delete it.
On Dec 22, 2009, at 6:13 PM, Robert Nichols <rnicholsNOSPAM at comcast.net> wrote:> Carlos Santana wrote: >> Hi, >> >> Does mount point specification while partitioning (order in which I >> specify /, /boot, swap etc..) affect performance? I am not sure about >> the syntax, but I guess one can also specify address/block range >> while >> partitioning. Does it affect IO performance? Probably a stupid >> question, but just curious.. Any insights? > > Not a stupid question at all. For ordinary disk drives the answer is > yes, absolutely. The outer tracks of a disk are physically longer, > and any but the most ancient of disk drives will pack more sectors > into those tracks. Since the disk rotates at a constant RPM, more > sectors per second pass under the head on the outer tracks. The > ratio of data rates for the outermost vs. innermost tracks is > typically 2:1 or a bit higher. Add to this the need for more and > longer seeks for filesystems on the inner tracks (again, less data > on each physical track), and the performance degrades even more. > > On most disks cylinder numbering starts at the outer tracks, but I > have heard of disks that number their cylinders in the opposite > direction -- never actually seen one, though.Also, for random IO the opposite is true, the rotational latency is significantly smaller on the inner tracks than the outer tracks, so random OPs perform better there. Though having different workloads on opposite sides of the disk is counter productive, but say you had one large volume for random IO workloads and another large volume for sequential workloads, you could allocate the beginning chunk to your most performance oriented servers on the sequential volume and the end chunk to your most performance oriented servers on the random volume. -Ross
Ross Walker wrote:> Also, for random IO the opposite is true, the rotational latency is > significantly smaller on the inner tracks than the outer tracks, so > random OPs perform better there. >um, most all hard disks are CAV, so the rotational latency measured in milliseconds is constant throughout the disk. usually 50% of a turn is the assumed mean rotational latency
On Dec 22, 2009, at 10:14 PM, John R Pierce <pierce at hogranch.com> wrote:> Ross Walker wrote: >> Also, for random IO the opposite is true, the rotational latency is >> significantly smaller on the inner tracks than the outer tracks, so >> random OPs perform better there. >> > > um, most all hard disks are CAV, so the rotational latency measured in > milliseconds is constant throughout the disk. usually 50% of a turn > is > the assumed mean rotational latencyI think you might be confusing CAV with CLV of optical drives. http://en.m.wikipedia.org/wiki/Constant_Angular_Velocity -Ross
On Wed, 2009-12-23 at 09:37 -0500, Ross Walker wrote:> <snip>> I think you might be confusing CAV with CLV of optical drives. > > http://en.m.wikipedia.org/wiki/Constant_Angular_Velocity > > -RossThat was my thought. However, I think most are missing the boat on this. I have always looked at the anticipated work profile on the drive and tried to place partitions to minimize seek time - that being the single biggest latency issue, IMO. By placing the most frequently accessed partitons adjacent to each other, and near the middle of the platter(s), seek delays are minimized. With the advent of LVM, I feel this is more easily fine tuned, initially and later after the /real/ workload can be statistically profiled. -- Bill