FAQ
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 at gmail.com

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  • John R Pierce at Feb 6, 2011 at 4:30 pm

    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.
  • Les Mikesell at Feb 7, 2011 at 1:05 am

    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 at gmail.com
  • Compdoc at Feb 6, 2011 at 4:50 pm
    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.
  • Nico Kadel-Garcia at Feb 6, 2011 at 8:46 pm

    On Sun, Feb 6, 2011 at 4:50 PM, 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.
    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.
  • Ray Van Dolson at Feb 6, 2011 at 10:51 pm

    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
  • Alexander Dalloz at Feb 7, 2011 at 3:55 am

    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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