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I bought a samsung 840 pro 250 GB ssd ~two months ago, and I was wondering how much has been written to it. Actually, I have four partitions: /,/home,boot, and swap. To figure out how much was written to home, I wrote

$cat /sys/fs/ext4/sda7/lifetime_write_kbytes

the output was 8,985,606,440 (I added the commas). So that's 8TB. I was wondering to myself, boy I must be writing a lot. I wonder how? I monitored how much I wrote from browsing the internet and other common tasks, then I remembered that I set up a daily cron to do fstrim, because the internet told me to. So I ran

$sudo fstrim -v /home

and the output was 606,591,360 bytes were trimmed.

Then I ran

$cat /sys/fs/ext4/sda7/lifetime_write_kbytes

again and sure enough I get 8,986,209,516. So my writes increased 600MB. Now this is not really enough to account for 8TB of writes, but I had just run fstrim a few minutes earlier, so I think fstrim usually trims a lot more than 600 MB. I will have to confirm this.

Regardless, do the writes caused by fstrim actually count towards decreasing the life of your SSD? I had the impression they didn't.

Edit

I don't have to guess how much is typically trimmed by the cron job. It turns out the internet told me to log the output of fstrim. Looking at the log, I see it trimmed an average of about 150 GB/day since Aug 24, 150 GB/day * 60 days is about 9 TB. So I think the fstrim does account for the 9TB of writes I have to my SSD.

Edit 2

smart was not enabled on the SSD, but I ran $sudo smartctl -s on /dev/sda7. Then I ran $sudo smartctl -a /dev/sda7. This was the output:

smartctl 5.43 2012-06-30 r3573 [x86_64-linux-3.8.0-25-generic] (local build)
Copyright (C) 2002-12 by Bruce Allen, http://smartmontools.sourceforge.net

=== START OF INFORMATION SECTION ===
Device Model:     Samsung SSD 840 PRO Series
Serial Number:    S1ATNSAD780426K
LU WWN Device Id: 5 002538 5a00ae961
Firmware Version: DXM05B0Q
User Capacity:    256,060,514,304 bytes [256 GB]
Sector Size:      512 bytes logical/physical
Device is:        Not in smartctl database [for details use: -P showall]
ATA Version is:   8
ATA Standard is:  ATA-8-ACS revision 4c
Local Time is:    Wed Oct 30 09:43:18 2013 CDT
SMART support is: Available - device has SMART capability.
SMART support is: Enabled

=== START OF READ SMART DATA SECTION ===
SMART overall-health self-assessment test result: PASSED

General SMART Values:
Offline data collection status:  (0x00) Offline data collection activity
                    was never started.
                    Auto Offline Data Collection: Disabled.
Self-test execution status:      (   0) The previous self-test routine completed
                    without error or no self-test has ever 
                    been run.
Total time to complete Offline 
data collection:        (53956) seconds.
Offline data collection
capabilities:            (0x53) SMART execute Offline immediate.
                    Auto Offline data collection on/off support.
                    Suspend Offline collection upon new
                    command.
                    No Offline surface scan supported.
                    Self-test supported.
                    No Conveyance Self-test supported.
                    Selective Self-test supported.
SMART capabilities:            (0x0003) Saves SMART data before entering
                    power-saving mode.
                    Supports SMART auto save timer.
Error logging capability:        (0x01) Error logging supported.
                    General Purpose Logging supported.
Short self-test routine 
recommended polling time:    (   2) minutes.
Extended self-test routine
recommended polling time:    (  20) minutes.
SCT capabilities:          (0x003d) SCT Status supported.
                    SCT Error Recovery Control supported.
                    SCT Feature Control supported.
                    SCT Data Table supported.

SMART Attributes Data Structure revision number: 1
Vendor Specific SMART Attributes with Thresholds:
ID# ATTRIBUTE_NAME          FLAG     VALUE WORST THRESH TYPE      UPDATED  WHEN_FAILED RAW_VALUE
  5 Reallocated_Sector_Ct   0x0033   100   100   010    Pre-fail  Always       -       0
  9 Power_On_Hours          0x0032   099   099   000    Old_age   Always       -       458
 12 Power_Cycle_Count       0x0032   099   099   000    Old_age   Always       -       28
177 Wear_Leveling_Count     0x0013   099   099   000    Pre-fail  Always       -       2
179 Used_Rsvd_Blk_Cnt_Tot   0x0013   100   100   010    Pre-fail  Always       -       0
181 Program_Fail_Cnt_Total  0x0032   100   100   010    Old_age   Always       -       0
182 Erase_Fail_Count_Total  0x0032   100   100   010    Old_age   Always       -       0
183 Runtime_Bad_Block       0x0013   100   100   010    Pre-fail  Always       -       0
187 Reported_Uncorrect      0x0032   100   100   000    Old_age   Always       -       0
190 Airflow_Temperature_Cel 0x0032   072   068   000    Old_age   Always       -       28
195 Hardware_ECC_Recovered  0x001a   200   200   000    Old_age   Always       -       0
199 UDMA_CRC_Error_Count    0x003e   100   100   000    Old_age   Always       -       0
235 Unknown_Attribute       0x0012   001   001   000    Old_age   Always       -       4294967148
241 Total_LBAs_Written      0x0032   099   099   000    Old_age   Always       -       748699270

SMART Error Log Version: 1
No Errors Logged

SMART Self-test log structure revision number 1
No self-tests have been logged.  [To run self-tests, use: smartctl -t]


SMART Selective self-test log data structure revision number 1
 SPAN  MIN_LBA  MAX_LBA  CURRENT_TEST_STATUS
    1        0        0  Not_testing
    2        0        0  Not_testing
    3        0        0  Not_testing
    4        0        0  Not_testing
    5        0        0  Not_testing
  255        0    65535  Read_scanning was never started
Selective self-test flags (0x0):
  After scanning selected spans, do NOT read-scan remainder of disk.
If Selective self-test is pending on power-up, resume after 0 minute delay.
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How often do you hit swap? If the answer is "more than almost never", that could easily account for your counts. –  ChrisInEdmonton Oct 30 '13 at 14:16
    
I set swappiness to 1 because the internet told me so. And I don't hibernate. Whenever I open system monitor, it says I am using 0 bytes of swap. –  NowIGetToLearnWhatAHeadIs Oct 30 '13 at 14:19
2  
My guess is that the TRIM commands sent by fstrim are being counted as writes by the ext4 driver, even though they aren't actually writes. What does SMART say? Run smartctl -a /dev/sdsa7 (you may have to install smartmontools first) and look at the Host_Writes* and NAND_Writes* fields. –  wingedsubmariner Oct 30 '13 at 14:26
    
@wingedsubmariner I didn't see the fields you mentioned, but please see edit 2. –  NowIGetToLearnWhatAHeadIs Oct 30 '13 at 15:39
    
I guess Samsung drives don't share that information. I do have those fields on my Intel. –  wingedsubmariner Oct 30 '13 at 15:52

1 Answer 1

up vote 0 down vote accepted

fstrim does not wear out your ssd; on the contrary, it increases its lifespan. The writes reported in lifetime_write_kbytes refer to file system writes at a higher abstraction level and don't necessarily correspond to physical writes to the drive. So although there are 40 TB of writes reported in lifetime_write_kbytes, running $sudo smartctl -s on /dev/sda7 followed by $sudo smartctl -a /dev/sda7 reveals that I have only written 750,000,000 blocks of 512 bytes each, which comes out to 375GB, which is a more reasonable value for two months of use.

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