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SSD Defrag ?



 
 
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  #16  
Old December 5th 18, 09:05 PM posted to alt.windows7.general
David E. Ross[_2_]
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Default 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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  #17  
Old December 5th 18, 09:41 PM posted to alt.windows7.general
Paul[_32_]
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Posts: 11,873
Default SSD Defrag ?

David E. Ross wrote:
On 12/4/2018 3:58 PM, Paul wrote:
XXXXXXX XXXXXXX XXX XXXXXXXXXXXX \____ Wasted time reduces
2us 2us 2us / aggregate bandwidth
(RAM Drive)


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


We were discussing what the penalty would be, if the
SSD was left *fragmented* and a defragmenter was *not*
used.

That's why I did the experiment, to measure the impact
of a ridiculous level of fragmentation (396000 fragments),
on the reading of a file from an SSD. There is still
quite good performance (383MB/sec ideal read speed, degrades
to 229MB/sec, on an Intel 545S SSD). On a regular hard
drive, the response would be horrible to behold (I could
test that, but I don't know if anyone cares).

Defragmentation of SSDs is not recommended, unless
the wear life is quite large. Maybe an Optane drive
would be a candidate for a defrag run (they're based
on something other than flash, and their granularity
is different). I think the time to read out a block
on Optane is 10us, while flash is 20us, and Samsung
did something to one of their flash drives, to get
their number down to 10us to try to catch up.

The fragmented files were generated by a C program,
opening two files, and alternating writes to the
two files. It turns out, that the Win10 OS I used
for the test, queues up writes a tiny bit, such that
the level of fragmentation wasn't as extreme as on other
OSes. I wasn't really in control of the 396000 number
in the 26GB test files. (My writes were 4096 bytes
each, but the average fragment is 65536 bytes long.)
That's what came out in the wash. Once the two files
are generated, I erase one file, to make it easier
to see the second file in the JKDefrag status window.
By taking an image of the partition, I can restore
it to other storage devices, and the restore "keeps"
the 396000 fragments.

Paul
 




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