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Old 12-22-2009, 10:13 PM
Robert Nichols
 
Default partitioning order and IO performance

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.

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Old 12-23-2009, 02:02 AM
Ross Walker
 
Default partitioning order and IO performance

On Dec 22, 2009, at 6:13 PM, Robert Nichols
<rnicholsNOSPAM@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

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Old 12-23-2009, 02:14 AM
John R Pierce
 
Default partitioning order and IO performance

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


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Old 12-23-2009, 01:37 PM
Ross Walker
 
Default partitioning order and IO performance

On Dec 22, 2009, at 10:14 PM, John R Pierce <pierce@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 latency

I think you might be confusing CAV with CLV of optical drives.

http://en.m.wikipedia.org/wiki/Constant_Angular_Velocity

-Ross

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Old 12-23-2009, 02:28 PM
"William L. Maltby"
 
Default partitioning order and IO performance

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
>
> -Ross

That 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

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Old 12-23-2009, 05:29 PM
John R Pierce
 
Default partitioning order and IO performance

Ross Walker wrote:
> I think you might be confusing CAV with CLV of optical drives.
> http://en.m.wikipedia.org/wiki/Constant_Angular_Velocity
>

no, I'm not. most HD's ('green drives' complicate this some) spin at
a constant RPM, so the rotational latency is the same on the inner and
outer tracks, an average of 1/2 turn, about 4mS for a 7200 rpm drive,
and 2mS for a 15000rpm enterprise drive . However, the data rate
changes. so the outer tracks have more data on them, which is read at a
higher speed in megabytes/second


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Old 12-23-2009, 05:36 PM
Timo Schoeler
 
Default partitioning order and IO performance

On 12/23/2009 07:29 PM, John R Pierce wrote:
> Ross Walker wrote:
>> I think you might be confusing CAV with CLV of optical drives.
>> http://en.m.wikipedia.org/wiki/Constant_Angular_Velocity
>>
>
> no, I'm not. most HD's ('green drives' complicate this some) spin at
> a constant RPM, so the rotational latency is the same on the inner and
> outer tracks, an average of 1/2 turn, about 4mS for a 7200 rpm drive,
> and 2mS for a 15000rpm enterprise drive . However, the data rate
> changes. so the outer tracks have more data on them, which is read at a
> higher speed in megabytes/second

That's why in ancient times one was setting up partitions so that the
swap area was the the beginning (mostly the outer tracks of the HD --
never hit a drive that did it the other way round) of the drive.

Try it yourself, get a spare HD and create three partitions on it, two
smaller ones at beginning/end of the drive, the third one filling the
gap between them; install bonnie++ and compare the transfer rates.

Timo
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Old 12-23-2009, 06:15 PM
Les Mikesell
 
Default partitioning order and IO performance

Timo Schoeler wrote:
> On 12/23/2009 07:29 PM, John R Pierce wrote:
>> Ross Walker wrote:
>>> I think you might be confusing CAV with CLV of optical drives.
>>> http://en.m.wikipedia.org/wiki/Constant_Angular_Velocity
>>>
>> no, I'm not. most HD's ('green drives' complicate this some) spin at
>> a constant RPM, so the rotational latency is the same on the inner and
>> outer tracks, an average of 1/2 turn, about 4mS for a 7200 rpm drive,
>> and 2mS for a 15000rpm enterprise drive . However, the data rate
>> changes. so the outer tracks have more data on them, which is read at a
>> higher speed in megabytes/second
>
> That's why in ancient times one was setting up partitions so that the
> swap area was the the beginning (mostly the outer tracks of the HD --
> never hit a drive that did it the other way round) of the drive.
>
> Try it yourself, get a spare HD and create three partitions on it, two
> smaller ones at beginning/end of the drive, the third one filling the
> gap between them; install bonnie++ and compare the transfer rates.

But these days, nothing should ever be reading from swap, although you
might write a bit there. If it does, buy some more RAM instead of
worrying about disk performance.

--
Les Mikesell
lesmikesell@gmail.com

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Old 12-23-2009, 10:55 PM
Ross Walker
 
Default partitioning order and IO performance

On Dec 23, 2009, at 1:29 PM, John R Pierce <pierce@hogranch.com> wrote:

> Ross Walker wrote:
>> I think you might be confusing CAV with CLV of optical drives.
>> http://en.m.wikipedia.org/wiki/Constant_Angular_Velocity
>>
>
> no, I'm not. most HD's ('green drives' complicate this some)
> spin at
> a constant RPM, so the rotational latency is the same on the inner and
> outer tracks, an average of 1/2 turn, about 4mS for a 7200 rpm drive,
> and 2mS for a 15000rpm enterprise drive . However, the data rate
> changes. so the outer tracks have more data on them, which is read
> at a
> higher speed in megabytes/second

You know your right. I don't know what I was thinking, a rotation is a
rotation and if it takes 4ms on the outer tracks then it takes 4ms on
the inner tracks.

It is I who had to two mixed up. It would be CLV that would make the
disk spin faster as it approached the inner tracks which is the only
way rotational latency would decrease.

Sorry for the noise, we now continue with your regularly scheduled
program.

-Ross

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Old 12-24-2009, 03:39 AM
Timo Schoeler
 
Default partitioning order and IO performance

On 12/23/2009 08:15 PM, Les Mikesell wrote:
> Timo Schoeler wrote:
>> On 12/23/2009 07:29 PM, John R Pierce wrote:
>>> Ross Walker wrote:
>>>> I think you might be confusing CAV with CLV of optical drives.
>>>> http://en.m.wikipedia.org/wiki/Constant_Angular_Velocity
>>>>
>>> no, I'm not. most HD's ('green drives' complicate this some) spin at
>>> a constant RPM, so the rotational latency is the same on the inner and
>>> outer tracks, an average of 1/2 turn, about 4mS for a 7200 rpm drive,
>>> and 2mS for a 15000rpm enterprise drive . However, the data rate
>>> changes. so the outer tracks have more data on them, which is read at a
>>> higher speed in megabytes/second
>>
>> That's why in ancient times one was setting up partitions so that the
>> swap area was the the beginning (mostly the outer tracks of the HD --
>> never hit a drive that did it the other way round) of the drive.
>>
>> Try it yourself, get a spare HD and create three partitions on it, two
>> smaller ones at beginning/end of the drive, the third one filling the
>> gap between them; install bonnie++ and compare the transfer rates.
>
> But these days, nothing should ever be reading from swap, although you
> might write a bit there. If it does, buy some more RAM instead of
> worrying about disk performance.

Sure, absolutely no question; *but* in the (ancient) times it was
important, it was 'nice' to have it as fast as possible, i.e. on the
fastest section(s) of the used HDs. So...

Timo
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