Other Can IOSTAT be trusted ? to report actual disk write

drsnx60

drsnx60

Hi
Trying to estimate how long a SSD device will live before failing .
if I have a Kingston A400 (cheap consumer grade ) 240GB SSD with a TBW value of 80 Terabyte.

80.000 / ( 365 x 3 x240 ) = 0.3 DiskWrites per day
240* 0,3 = 73

so for three years the A400 is listed to be able to receive 73 Gbytes per day.
and for five years the A400 is listed to be able to receive 44 Gbytes per day.

is there a known method to measure how much data is written to and deleted from a storage device during a delta T. ?
will IOSTAT tell me the truth on amount of Megabytes written to the medium during the interval set ?
Does IOSTAT include all data written to /dev/nda0 ? /dev/nda1 ......
Swap data ? UFS journal ? ZFS LOG & CACHE ?

what traffic to the storage device is not measured by IOSTAT ?
 
iostat will report writes as they happen through the VFS subsystem. That means it can count too many writes when some writes are superseded by new writes to the same location and never make it out of the VM cache.

Good SSDs have a direct counter for blocks written.
 
SMART Attribute ID 241 "Lifetime Writes from Host System" will give you total GB written on the disk and ID 177 will give you the wear levels
 
232 Available_Reservd_Space 0x0033 100 100 004 Pre-fail Always - 100
233 Media_Wearout_Indicator 0x0032 100 100 --- Old_age Always - 310
Click to expand...
These are the values I'd look at on an SSD. No worries here, as 100 is still a long ways from 004.
 
cracauer@ said:
iostat will report writes as they happen through the VFS subsystem. That means it can count too many writes when some writes are superseded by new writes to the same location and never make it out of the VM cache.
Click to expand...
Not sure if this is true. I think that iostat takes stats from devstat(9) and that takes input at the the storage layer (e.g., commonly CAM).
 
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