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Re: question about parallelism in cp command

From: L A Walsh
Subject: Re: question about parallelism in cp command
Date: Fri, 28 Jun 2019 10:35:59 -0700
User-agent: Thunderbird

On 2019/06/28 04:52, Marc Roos wrote:
> There are always exceptions like with clustered filesystem etc etc. That 
> is why I wrote 'most used'. If you take all the issued 'cp' commands of 
> today in the world. I would bet 80%-95% of them would not benefit from 
> some sort of parallel processing.
        Single disks already benefit from some parallel processing, and
could benefit more as the write process and the on-disk cache process
is increased.  That's why many hard disks are moving to a SSD+HD combo
with a NVMe SSD being able to handle near-memory speeds of up to

        The benefits of parallelism involve being able to order
reads and writes to minimize the need for disk seeking and start/stop
overhead and moving to disk streaming of tracks where disk speeds can
begin to reach I/O transfer limits.  

        With higher capacities, comes a higher write speed since the disk
run at the same linear speed / technology.  The idea is for "cp -r" to 
take 1/10th the iops to copy the same data due to it being re-organizable
by the OS and by drivers, but that can only be done if all of the data on
tracks can be rewritten.  Since the data on tracks rarely even comes from the
same file, you need multiple threads to 1) read all the separate files storing
data in a track, 2) write all the separate files storing something in the

        The key is scaling memory usage to allow for a thread to completely
fill its memory buffer between reads or writes to the device.  Unfortunately, 
cp rarely uses the memory it could due to concerns of voiding some cache that
may be used sometime in the future...someday.  A tunable might be deciding how 
much memory to allocate to something like cp, it could write out entire 
files in 1 iop (if the driver allows).  

        This type of throughput might involve regular defragmenting of disks
to allow multiple files transfered to/from disk at once if they were all small
enough to fit on, say 1 track, but to do that a demand for all of those files
needs to be there for underlying fs-drivers to r/w multiple full tracks
at a time while performing only 1 iop to write multiple tracks.

> -----Original Message-----
> From: L A Walsh [mailto:address@hidden] 
> Sent: vrijdag 28 juni 2019 13:15
> To: Marc Roos
> Cc: aglo; coreutils
> Subject: Re: question about parallelism in cp command
> On 2019/06/06 09:25, Marc Roos wrote:
>> Hmmm without being a maintainer. I would say cp -r is most used on 
>> single disk, so one thread is using the maximum disk iops taking y 
>> time to copy.
> ---
>     not exactly true, if the 1 disk as a 20 disk raid10.
> You can target 10 areas at a time and get considerable benefit if they 
> are spread across multiple disks in the raid.

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