From owner-svn-src-all@freebsd.org  Fri Oct 16 13:19:49 2015
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Date: Fri, 16 Oct 2015 16:19:40 +0300
From: Slawa Olhovchenkov <slw@zxy.spb.ru>
To: Warner Losh <imp@FreeBSD.org>
Cc: src-committers@freebsd.org, svn-src-all@freebsd.org,
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Subject: Re: svn commit: r289405 - head/sys/ufs/ffs
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On Fri, Oct 16, 2015 at 03:06:02AM +0000, Warner Losh wrote:

> Author: imp
> Date: Fri Oct 16 03:06:02 2015
> New Revision: 289405
> URL: https://svnweb.freebsd.org/changeset/base/289405
> 
> Log:
>   Do not relocate extents to make them contiguous if the underlying drive can do
>   deletions. Ability to do deletions is a strong indication that this
>   optimization will not help performance. It will only generate extra write
>   traffic. These devices are typically flash based and have a limited number of
>   write cycles. In addition, making the file contiguous in LBA space doesn't
>   improve the access times from flash devices because they have no seek time.

In reality, flash devices have seek time, about 0.1ms.
Many flash devices can do 8 simultaneously "seek" (I think NVMe can do
more).