SSD Defrag ?

On 12/4/2018 3:58 PM, Paul wrote:
David E. Ross wrote:
On 12/4/2018 3:08 PM, Paul wrote:
wrote:
Appreciate all the info, but not sure I have a clear answer.

Is there no issue at all, accessing a very fragmented file, compared
to accessing an unfragmented file, on an SSD ?
Here are some test results.

https://i.postimg.cc/ry7VnwF7/fragmentation.gif

26GB test file

Checksum used as a read test of the file.

Fragmented file has around 396,000 fragments.
Unfragmented file is contiguous.

On a RAMDISK (close to zero seek) 970MB/sec fragmented read
993MB/sec unfragmented read

On an SSD where a 20usec seek time 229MB/sec fragmented read
is present, so 396000 of the 20usec 383MB/sec unfragmented read
seeks are required.

That's to give some idea how much effect
an extremely fragmented file would have
on an SSD.

Paul

A fragmented drive takes longer to read than an unfragments
(defragmented) drive???

Yes, the extra time to seek from one chunk of the file
to the next.

XXXXXXX XXXXXXX XXX XXXXXXXXXXXX \____ Wasted time reduces
20us 20us 20us / aggregate bandwidth
(SSD)
On a hard drive, it's much worse. Head
switches cost 1ms, seeks are more expensive.

XXXXXXX XXXXXXX XXX XXXXXXXXXXXX \____ Wasted time reduces
8ms 8ms 8ms / aggregate bandwidth
(HDD)
On my RAMDisk (I haven't measured it), it should
be around these numbers. These would be typical
numbers for a "hardware" RAM drive. The OS software
stack probably degrades numbers like this quite
a bit.

XXXXXXX XXXXXXX XXX XXXXXXXXXXXX \____ Wasted time reduces
2us 2us 2us / aggregate bandwidth
(RAM Drive)
HTH,
Paul


Your two examples illustrate fragmentation. Unfragmentation is obtained
by deframenting, which yields files with their contents contiguous.

--
David E. Ross
http://www.rossde.com/

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