Let's think about the read and write advantages with very large block sizes.

With small (default 512 byte) reads you get extreme overhead with modern
disks. With older disks you got one disk block per read transaction. "Way
back when" the disk read time was actually large compared to the transaction
time. So the real win for issuing reads in larger sizes came from rotational
latency wins. In the late 80s I discovered there were serious advantages up
to between 4 and 8 blocks per transaction with Adaptec SCSI to ST-506 boards
and straight SCSI disks. So I built a transaction read ahead buffer that
amounted to 32 blocks. This improved compile times to a most gratifying
degree. As time went by the compile type performance did not improve much
with larger buffers. But issuing larger reads made a difference up to about
65536 bytes. So I habitually worked copies with 131072 byte buffers. Later
on it became profitable with "dd" copies on 'ix to go up to about a megabyte
per block. At that size the transaction delays were on the order of the
read/write times.

Note that this is on systems with hundreds to thousands of megabytes of
RAM and are lightly loaded so there is a chance to find a contiguous
buffer allowing transfers at the actual SCSI/SATA level to be singular. If
you try buffers that are too large they are generally not contiguous and
get slowed down as they are broken into multiple transactions due to the
usual levels of memory fragmentation.

These days you might be able to go as high as 100 megabytes for buffers if
you want transaction delays to be small compared to the actual disk read/write
times. Past that you win too little with respect to speed. So I for one am
sitting here reading this thread with a bemused expression wondering where
the real win is with 2 gigabyte buffers let alone 12 gigabyte buffers. That
is probably why the OS might have a practical read buffer size of 2 gigabytes.

(On a heavily loaded system, just when are you going to find 12 gigabytes
of fully contiguous storage?)

{^_^}

On 2012/02/06 09:24, Chris Schanzle wrote:
> It's a shame the original question didn't explain what and why he was trying to
> do something with these large blocks.
>
> Huge block sizes are useful if you have lots of ram and are copying very large
> files on the same set of spindles. This minimizes disk seeking caused by head
> repositioning for reads and writes and is vastly more efficient than say, "cp"
> which often uses at most 32 KB reads/writes and relies on the VM system to flush
> the writes (buffered by dirtying memory pages) pages as it deems appropriate
> (tunables in /proc/sys/vm/dirty*).
>
> Anyway, let's look at what system calls 'dd' does:
>
> $ strace dd if=/dev/zero of=/dev/shm/deleteme bs=12G count=1
> ...
> open("/dev/shm/deleteme", O_WRONLY|O_CREAT|O_TRUNC, 0666) = 3
> dup2(3, 1) = 1
> close(3) = 0
> mmap(NULL, 12884914176, PROT_READ|PROT_WRITE, MAP_PRIVATE|MAP_ANONYMOUS, -1, 0)
> = 0x2af98c7a0000
> read(0, "\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0"...,
> 12884901888) = 2147479552
> write(1, "\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0"...,
> 2147479552) = 2147479552
> close(0) = 0
> close(1) = 0
> ...
>
> (count=2 is also interesting)
>
> Things to notice:
>
> 1. strace shows dd is issuing a 12GB read from the input descriptor (/dev/zero)
> but is getting a 'short read' from the kernel of 2GB. Short reads are not an error.
>
> 2. The "count=" option in the dd man page specifies that it limits the number of
> INPUT blocks. So it writes what it read (2GB) and quits.
>
> So it seems to be working as designed, though perhaps not as you want.
>
> Adding 'iflag=fullblock' will cause dd to perform multiple reads to fill the
> input block size.
>
> mmap(NULL, 12884914176, PROT_READ|PROT_WRITE, MAP_PRIVATE|MAP_ANONYMOUS, -1, 0)
> = 0x2b2d8735e000
> read(0, "\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0"...,
> 12884901888) = 2147479552
> read(0, "\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0"...,
> 10737422336) = 2147479552
> read(0, "\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0"...,
> 8589942784) = 2147479552
> read(0, "\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0"...,
> 6442463232) = 2147479552
> read(0, "\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0"...,
> 4294983680) = 2147479552
> read(0, "\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0"...,
> 2147504128) = 2147479552
> read(0, "\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0"...,
> 24576) = 24576
> write(1, "", 12884901888) = 2147479552
> write(1, "", 10737422336) = 2147479552
> write(1, "\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0"...,
> 8589942784) = 2147479552
> write(1, "", 6442463232) = 2147479552
> write(1, "\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0"...,
> 4294983680) = 2147479552
>
> Notice how the writes empty the input 2GB at a time.
>
> Of course, all this reading/writing goes through typical VM buffering, so you
> might want to consider direct i/o: iflag=direct and oflag=direct.
>
> Which begs the question: how to encourage the kernel to allow larger read/write
> file buffers? Couldn't find that answer easily. Anyone?
>
> -c
>
> On 02/02/2012 12:32 PM, Stephen J. Gowdy wrote:
>> Hi Andrey,
>> Why would you want a block size in GB? I don't know what the
>> actual limit for dd itself is, although it does seem to be exactly 2GiB.
>>
>> regards,
>>
>> Stephen.
>>
>> On Thu, 2 Feb 2012, Andrey Y. Shevel wrote:
>>
>>>
>>> Hi Stephen,
>>>
>>> thank you for your reply.
>>>
>>> ======
>>> [root@pcfarm-10 ~]# rpm -qa --queryformat "%{NAME}-%{VERSION}.%{ARCH}\n" |
>>> grep coreutils
>>> policycoreutils-1.33.12.x86_64
>>> policycoreutils-newrole-1.33.12.x86_64
>>> coreutils-5.97.x86_64
>>> policycoreutils-gui-1.33.12.x86_64
>>> =====
>>>
>>> And obviously
>>>
>>> ================
>>> [root@pcfarm-10 ~]# arch
>>> x86_64
>>> ===============
>>>
>>>
>>> The result is prety same as I shown earlier.
>>>
>>> And the same I see at CERN
>>>
>>> =======================
>>> [lxplus427] /afs/cern.ch/user/s/shevel> dd if=/dev/zero of=/tmp/testx64
>>> bs=3GB count=1
>>> 0+1 records in
>>> 0+1 records out
>>> 2147479552 bytes (2.1 GB) copied, 5.91242 seconds, 363 MB/s
>>> [lxplus427] /afs/cern.ch/user/s/shevel> rpm -q --file /bin/dd
>>> coreutils-5.97-34.el5
>>> [lxplus427] /afs/cern.ch/user/s/shevel> rpm -qa --queryformat
>>> "%{NAME}-%{VERSION}.%{ARCH}\n" | grep coreutil
>>> policycoreutils-1.33.12.x86_64
>>> coreutils-5.97.x86_64
>>> policycoreutils-gui-1.33.12.x86_64
>>> ===========================
>>>
>>>
>>>
>>>
>>>
>>> As far as I understand the main question is "is there 64 bit dd version which
>>> can operate more then 2GB value for BS in SL anyway?"
>>>
>>> Any answer (yes or no) is good to know.
>>>
>>> Many thanks,
>>>
>>> Andrey
>>>
>>>
>>> On Wed, 1 Feb 2012, Stephen J. Gowdy wrote:
>>>
>>>> Date: Wed, 1 Feb 2012 19:10:14 +0100 (CET)
>>>> From: Stephen J. Gowdy<[log in to unmask]>
>>>> To: Andrey Y. Shevel<[log in to unmask]>
>>>> Cc: [log in to unmask]
>>>> Subject: Re: coreutils for 64 bit
>>>>
>>>> Exactly.... if you type "man rpm" it will show you how you get it to print
>>>> the arch string (usually i686 or x86_64). Since you seem unabel to read a
>>>> man page what you want to type is;
>>>>
>>>> rpm -qa --queryformat "%{NAME}-%{VERSION}.%{ARCH}\n" | grep coreutils
>>>>
>>>> (or miss out the VERSION if you want to see somethign similar to yum)
>>>>
>>>> On Wed, 1 Feb 2012, Andrey Y. Shevel wrote:
>>>>
>>>>>
>>>>> Hi Stephen,
>>>>>
>>>>> thanks for the reply.
>>>>>
>>>>> I am not sure that I do understand you (sorry for my stupidity).
>>>>>
>>>>> I have
>>>>> =======================================
>>>>> [root@pcfarm-10 ~]# yum list | grep coreutil
>>>>> Failed to set locale, defaulting to C
>>>>> coreutils.x86_64 5.97-34.el5 installed
>>>>> policycoreutils.x86_64 1.33.12-14.8.el5 installed
>>>>> policycoreutils-gui.x86_64 1.33.12-14.8.el5 installed
>>>>> policycoreutils-newrole.x86_64 1.33.12-14.8.el5 installed
>>>>> [root@pcfarm-10 ~]# rpm -q --file /bin/dd
>>>>> coreutils-5.97-34.el5
>>>>> =============================================
>>>>>
>>>>> Presumably all packages are appropriate (they have suffix x86_64) as
>>>>> shown
>>>>> by yum.
>>>>>
>>>>> At the same time rpm does show packages without above suffixes
>>>>>
>>>>> =========================
>>>>> [root@pcfarm-10 ~]# rpm -qa | grep coreutil
>>>>> policycoreutils-1.33.12-14.8.el5
>>>>> policycoreutils-newrole-1.33.12-14.8.el5
>>>>> coreutils-5.97-34.el5
>>>>> policycoreutils-gui-1.33.12-14.8.el5
>>>>> =========================
>>>>>
>>>>>
>>>>>
>>>>>
>>>>> On Wed, 1 Feb 2012, Stephen J. Gowdy wrote:
>>>>>
>>>>>> Date: Wed, 1 Feb 2012 11:32:40 +0100 (CET)
>>>>>> From: Stephen J. Gowdy<[log in to unmask]>
>>>>>> To: Andrey Y Shevel<[log in to unmask]>
>>>>>> Cc: [log in to unmask]
>>>>>> Subject: Re: coreutils for 64 bit
>>>>>>> It says it only copied 2.1GB. You are runnig a 64bit OS. You
>>>>> reinstalld> the same coreutils package. You need to change the format of
>>>>> the package> names from "rpm -qa" if you want to see the architecture
>>>>> ("man rpm"> should help you figure out how).
>>>>>>> On Wed, 1 Feb 2012, Andrey Y Shevel wrote:
>>>>>>>> Hi,
>>>>>>>>> I just paid attention that utility 'dd' uses just 2 GB even I
>>>>> use> > greater
>>>>>>> block size (BS). For example
>>>>>>>>> =====
>>>>>>> [root@pcfarm-10 ~]# dd if=/dev/zero of=/mnt/sdb/TestFile-S1 bs=12GB
>>>>>>> count=1
>>>>>>> 0+1 records in
>>>>>>> 0+1 records out
>>>>>>> 2147479552 bytes (2.1 GB) copied, 15.8235 seconds, 136 MB/s
>>>>>>> ============
>>>>>>>>> BTW,
>>>>>>>>> [root@pcfarm-10 ~]# uname -a
>>>>>>> Linux pcfarm-10.pnpi.spb.ru 2.6.18-274.17.1.el5xen #1 SMP Tue Jan 10
>>>>>>> 16:41:16 EST 2012 x86_64 x86_64 x86_64 GNU/Linux
>>>>>>> [root@pcfarm-10 ~]# cat /etc/issue
>>>>>>> Scientific Linux SL release 5.7 (Boron)
>>>>>>> Kernel \r on an \m
>>>>>>>>>>>>>>> I decided to reinstall coreutils:
>>>>>>>>> [root@pcfarm-10 ~]# yum reinstall coreutils.x86_64
>>>>>>> Failed to set locale, defaulting to C
>>>>>>> Loaded plugins: kernel-module
>>>>>>> Setting up Reinstall Process
>>>>>>> Resolving Dependencies
>>>>>>> --> Running transaction check
>>>>>>> ---> Package coreutils.x86_64 0:5.97-34.el5 set to be updated
>>>>>>> --> Finished Dependency Resolution
>>>>>>> Beginning Kernel Module Plugin
>>>>>>> Finished Kernel Module Plugin
>>>>>>>>> Dependencies Resolved
>>>>>>>>>
>>>>> ===========================================================================================
>>>>>
>>>>>>> Package Arch Version
>>>>>>> Repository
>>>>>>> Size
>>>>>>>
>>>>> ===========================================================================================
>>>>>
>>>>>>> Reinstalling:
>>>>>>> coreutils x86_64 5.97-34.el5> > sl-base
>>>>>>> 3.6 M
>>>>>>>>> Transaction Summary
>>>>>>>
>>>>> ===========================================================================================
>>>>>
>>>>>>> Remove 0 Package(s)
>>>>>>> Reinstall 1 Package(s)
>>>>>>> Downgrade 0 Package(s)
>>>>>>>>> Total download size: 3.6 M
>>>>>>> Is this ok [y/N]: y
>>>>>>> Downloading Packages:
>>>>>>> coreutils-5.97-34.el5.x86_64.rpm
>>>>> |> > 3.6
>>>>>>> MB
>>>>>>> 00:05
>>>>>>> Running rpm_check_debug
>>>>>>> Running Transaction Test
>>>>>>> Finished Transaction Test
>>>>>>> Transaction Test Succeeded
>>>>>>> Running Transaction
>>>>>>> Installing : coreutils
>>>>>>> 1/1
>>>>>>>>> Installed:
>>>>>>> coreutils.x86_64 0:5.97-34.el5
>>>>>>>>>>> Complete!
>>>>>>> =========================
>>>>>>>>> However after that I see
>>>>>>>>>>> [root@pcfarm-10 ~]# ls -l /bin/dd
>>>>>>> -rwxr-xr-x 1 root root 41464 Jul 26 2011 /bin/dd
>>>>>>> [root@pcfarm-10 ~]# rpm -q --file /bin/dd
>>>>>>> coreutils-5.97-34.el5
>>>>>>>>>>> [root@pcfarm-10 ~]# rpm -qa | grep coreutils
>>>>>>> policycoreutils-1.33.12-14.8.el5
>>>>>>> policycoreutils-newrole-1.33.12-14.8.el5
>>>>>>> coreutils-5.97-34.el5
>>>>>>> policycoreutils-gui-1.33.12-14.8.el5
>>>>>>>>>>> i.e. no package with name coreutils.x86_64
>>>>>>>>> I failed to find anything on the topic in scientific linux
>>>>> mailing> > list.
>>>>>>>>> Does somebody know about dd for 64 bit ?
>>>>>>>>> Many thanks in advance,
>>>>>>>>> Andrey
>>>>>>>>>
>>>>>
>>>>
>>>>
>>>
>>>
>>
>> --
>> /------------------------------------+-------------------------\
>> |Stephen J. Gowdy | CERN Office: 8-1-11|
>> |http://cern.ch/gowdy/ | CH-1211 Geneva 23 |
>> | | Switzerland |
>> |EMail: [log in to unmask] | Tel: +41 76 487 2215 |
>> \------------------------------------+-------------------------/
>