svn commit: r289405 - head/sys/ufs/ffs

[Warner Losh]

> On Oct 16, 2015, at 7:19 AM, Slawa Olhovchenkov <slw at zxy.spb.ru> wrote:
> 
> 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).

That’s just not true. tREAD for most flash is a few tens of microseconds. The
streaming time is at most 10 microseconds. There’s no “seek” time in the classic
sense. Once you get the data, you have it. There’s no extra “read time” in
the NAND flash parts.

And the number of simultaneous reads depends a lot on how the flash vendor
organized the flash. Many of today’s designs use 8 or 16 die parts that have 2
to 4 planes on them, giving a parallelism in the 16-64 range. And that’s before
we get into innovative strategies that use partial page reads to decrease tREAD
time and novel data striping methods.

Seek time, as a separate operation, simply doesn’t exist.

Furthermore, NAND-based devices are log-structured with garbage collection
for both retention and to deal with retired blocks in the underlying NAND. The
relationship between LBA ranges and where the data is at any given time on
the NAND is almost uncorrelated.

So, rearranging data so that it is in LBA contiguous ranges doesn’t help once
you’re above the FFS block level.

Warner

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