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Old 02-06-2011, 08:14 PM
Les Mikesell
 
Default RHEL/Centos6 handling disks w/4k sectors?

Does anyone know if 4k sectors will be handled better by the kernel in Centos6?
I'd like to copy backups to a 750Gb laptop type drive for offsite storage but
the best write speed I can get is about 8MB/sec even with dd to the raw disk
which shouldn't have an issue with partition alignment.

--
Les Mikesell
lesmikesell@gmail.com


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Old 02-06-2011, 08:30 PM
John R Pierce
 
Default RHEL/Centos6 handling disks w/4k sectors?

On 02/06/11 1:14 PM, Les Mikesell wrote:
> Does anyone know if 4k sectors will be handled better by the kernel in Centos6?
> I'd like to copy backups to a 750Gb laptop type drive for offsite storage but
> the best write speed I can get is about 8MB/sec even with dd to the raw disk
> which shouldn't have an issue with partition alignment.


what are you using on dd as the blocksize (bs=) ? should be a fairly
large multiple of 4K, like 1048576 (eg, 1MB writes).

I get 50-80MB/sec writes to a laptop drive hooked up to my windows
desktop when I'm cloning said laptop drives. if you're only seeing
8MB, something is very wrong.


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Old 02-06-2011, 08:50 PM
"compdoc"
 
Default RHEL/Centos6 handling disks w/4k sectors?

> the best write speed I can get is about 8MB/sec


A while back I researched 4k sector drives since most new drives have them
now. There is a problem with speed if you get the partition wrong.

The answer seems to be to creating a partition with 1 meg of unpartitioned
space preceding the first partition. This causes sector 2048 to be the first
sector of the partition, and avoids the problem.

Newer versions of gparted do this for you automatically when you use it to
create a partition.



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Old 02-07-2011, 12:46 AM
Nico Kadel-Garcia
 
Default RHEL/Centos6 handling disks w/4k sectors?

On Sun, Feb 6, 2011 at 4:50 PM, compdoc <compdoc@hotrodpc.com> wrote:
>> the best write speed I can get is about 8MB/sec
>
>
> A while back I researched 4k sector drives since most new drives have them
> now. There is a problem with speed if you get the partition wrong.
>
> The answer seems to be to creating a partition with 1 meg of unpartitioned
> space preceding the first partition. This causes sector 2048 to be the first
> sector of the partition, and avoids the problem.
>
> Newer versions of gparted do this for you automatically when you use it to
> create a partition.

This turns out to also be a significant issue with RHEL/CentOS 5 and
virtualized OS images, where the upstream repository has 4096 byte
blocks, whether NetApp or some other architecture. NetApp images in
VMWare get.... unhappy if you use the standard allocated 63 blocks for
the BIOS and partition records. They're much, much happer if the
begging of the first logical volume begins at block 65, but there's no
graceful way to set this up in anaconda or most configuration tools.
You have to pre-script it with parted.
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Old 02-07-2011, 02:51 AM
Ray Van Dolson
 
Default RHEL/Centos6 handling disks w/4k sectors?

On Sun, Feb 06, 2011 at 02:50:34PM -0700, compdoc wrote:
> > the best write speed I can get is about 8MB/sec
>
>
> A while back I researched 4k sector drives since most new drives have them
> now. There is a problem with speed if you get the partition wrong.
>
> The answer seems to be to creating a partition with 1 meg of unpartitioned
> space preceding the first partition. This causes sector 2048 to be the first
> sector of the partition, and avoids the problem.
>
> Newer versions of gparted do this for you automatically when you use it to
> create a partition.

Just did a SL 6.0 install on a machine with two Hitachi 2TB drives with
4K sectors. parted warned me that the default values I'd chosen
weren't optimal for the sector size and I adjusted manually.

Note that I set these drives up post-install -- no idea if Anaconda
would have done the "right thing" or not.

Ray
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Old 02-07-2011, 05:05 AM
Les Mikesell
 
Default RHEL/Centos6 handling disks w/4k sectors?

On 2/6/11 3:30 PM, John R Pierce wrote:
>
>> Does anyone know if 4k sectors will be handled better by the kernel in Centos6?
>> I'd like to copy backups to a 750Gb laptop type drive for offsite storage but
>> the best write speed I can get is about 8MB/sec even with dd to the raw disk
>> which shouldn't have an issue with partition alignment.
>
>
> what are you using on dd as the blocksize (bs=) ? should be a fairly
> large multiple of 4K, like 1048576 (eg, 1MB writes).
>
> I get 50-80MB/sec writes to a laptop drive hooked up to my windows
> desktop when I'm cloning said laptop drives. if you're only seeing
> 8MB, something is very wrong.

Not all laptop drives use 4k sectors - probably just 750Gb and up.

I was using obs=4M. I thought there was more to it than alignment - and changes
were made in more current kernels to help with the speed.

--
Les Mikesell
lesmikesell@gmail.com
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Old 02-07-2011, 07:55 AM
Alexander Dalloz
 
Default RHEL/Centos6 handling disks w/4k sectors?

Am 07.02.2011 04:51, schrieb Ray Van Dolson:
> On Sun, Feb 06, 2011 at 02:50:34PM -0700, compdoc wrote:
>>> the best write speed I can get is about 8MB/sec
>>
>>
>> A while back I researched 4k sector drives since most new drives have them
>> now. There is a problem with speed if you get the partition wrong.
>>
>> The answer seems to be to creating a partition with 1 meg of unpartitioned
>> space preceding the first partition. This causes sector 2048 to be the first
>> sector of the partition, and avoids the problem.
>>
>> Newer versions of gparted do this for you automatically when you use it to
>> create a partition.
>
> Just did a SL 6.0 install on a machine with two Hitachi 2TB drives with
> 4K sectors. parted warned me that the default values I'd chosen
> weren't optimal for the sector size and I adjusted manually.
>
> Note that I set these drives up post-install -- no idea if Anaconda
> would have done the "right thing" or not.
>
> Ray

RHEL 6's anaconda auto-aligns properly by starting at sector 2048.

Alexander
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