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Drive size is not rounded up

#1

D

Dagannoth

I have a couple of strange little SSDs whose labels show 16 GB, but actually fall a little short. One of them is 15,837,691,904 bytes [15.8 GB], and the other is 15,934,586,880 bytes [15.9 GB]. ReadSpeed and SR-R02 both show this as 15 GB instead of rounding up to 16. This is a tiny cosmetic issue, so no hard feelings if it doesn't get fixed. Just wanted to share this little quirk :)

ReadSpeed log:
Code:
  +--------------------------------------------------------------------------+
  | ReadSpeed: Hyper-accurate mass storage read-performance benchmark. rel 1 |
  |  Benchmarked values are in megabytes read per second at five locations.  |
  +--------------------------------------------------------------------------+

   NOTE: This system contains an NVMe controller which is not yet supported.
         The addition of full NVMe support =IS= on our development roadmap.

Driv Size  Drive Identity     Location:    0      25%     50%     75%     100
---- ----- ---------------------------- ------- ------- ------- ------- -------
81   15GB Virtium - StorFly VSFA25C      92.8    89.2    88.3    87.1    91.9

                  Benchmarked: Tuesday, 2021-03-09 at 04:50
-----------------------------------------------------------------------------
   See the ReadSpeed forums at forums.grc.com for help and community support.

SR-R02:
SR-R0201.png


smartctl:
Code:
smartctl 7.1 2019-12-30 r5022 [x86_64-linux-5.5.4-pmagic64] (local build)
Copyright (C) 2002-19, Bruce Allen, Christian Franke, www.smartmontools.org

=== START OF INFORMATION SECTION ===
Device Model:     Virtium - StorFly VSFA25CC016G-200
Serial Number:    P1T05003893304120603
Firmware Version: 0812-000
User Capacity:    15,934,586,880 bytes [15.9 GB]
Sector Size:      512 bytes logical/physical
Rotation Rate:    Solid State Device
Device is:        Not in smartctl database [for details use: -P showall]
ATA Version is:   ACS-2 (minor revision not indicated)
Local Time is:    Sun Feb 28 02:48:42 2021 MST
SMART support is: Available - device has SMART capability.
SMART support is: Enabled


#2

Mervyn Haynes

Mervyn Haynes

Actually if you divide the bytes by 1024 you get 15.47 & 15.56! Not sure if this helps though.


#3

D

Dagannoth

Drive manufacturers use 1 KB = 1,000 bytes. Here's what my 128 GB Samsung PM830 shows:

ReadSpeed:
Code:
  +--------------------------------------------------------------------------+
  | ReadSpeed: Hyper-accurate mass storage read-performance benchmark. rel 1 |
  |  Benchmarked values are in megabytes read per second at five locations.  |
  +--------------------------------------------------------------------------+

Driv Size  Drive Identity     Location:    0      25%     50%     75%     100
---- ----- ---------------------------- ------- ------- ------- ------- -------
 82  128GB SAMSUNG SSD PM830 2.5" 7m     476.3   481.0   480.9   481.0   448.7

                  Benchmarked: Tuesday, 2017-05-16 at 17:45
 -----------------------------------------------------------------------------
   See the ReadSpeed forums at forums.grc.com for help and community support.

smartctl:
Code:
smartctl 7.1 2019-12-30 r5022 [x86_64-linux-5.5.4-pmagic64] (local build)
Copyright (C) 2002-19, Bruce Allen, Christian Franke, www.smartmontools.org

=== START OF INFORMATION SECTION ===
Model Family:     Samsung based SSDs
Device Model:     SAMSUNG SSD PM830 2.5" 7mm 128GB
Serial Number:    S0TYNSBCB03775
LU WWN Device Id: 5 002538 043584d30
Firmware Version: CXM03D1Q
User Capacity:    128,035,676,160 bytes [128 GB]
Sector Size:      512 bytes logical/physical
Rotation Rate:    Solid State Device
Device is:        In smartctl database [for details use: -P show]
ATA Version is:   ATA8-ACS T13/1699-D revision 4c
SATA Version is:  SATA 3.0, 6.0 Gb/s (current: 6.0 Gb/s)
Local Time is:    Sat Feb 13 20:33:38 2021 MST
SMART support is: Available - device has SMART capability.
SMART support is: Enabled


#4

P

PHolder

both show this as 15 GB instead of rounding up to 16.
Perhaps @Steve doesn't round them at all, but simply truncates to a whole number? If that's what he did I would suspect it would be because it would be common for most drives to have more bytes than rated, not less? Such as your 128G being 128.04.


#5

Steve

Steve

Huh. Good find, @Dagannoth. Thanks!! Normally I do and would round... but I just checked, and I think the reason I didn't was that I was performing a double-length 64-bit division by 1000 (not 1024). And it didn't occur to me to check for a remainder for rounding. I'll definitely want to do that! Thanks!!