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Re: CF Card wear optimalisation for ext4



On Oct 16, 2014, at 10:25 AM, Bodo Thiesen <bothie gmx de> wrote:
> * Andreas Dilger <adilger dilger ca> hat geschrieben:
> 
>> You can see in the ext4 superblock the amount of data that has been
>> written to a filesystem over its lifetime:
>> 
>> Note that this number isn't wholly accurate, but rather a guideline.
> 
> Is is more like a completely bogus value at best:
> 
> # LANG=C df -h / | grep root
> /dev/root       3.7T  3.6T   73G  99% /
> # grep [0-9] /proc/partitions
>   8        0 3907018584 sda
> # tune2fs -l /dev/sda | grep Lifetime
> Lifetime writes:          2503 GB
> 
> 3.7 TB Disk/Partition, 3.6 TB space in use but only 2.4 TB writes.
> 
> No, there are no 1.2 TB + x allocated but never written to clusters on
> that file system.
> 
> And if /sys/fs/ext4/*/*_write_kbytes are as correct as the "Lifetime
> writes" value, than the correct answer to Jelle's question is: "There is
> no way currently to figure out the actual number of writes to a device".

The "lifetime writes" value has not been around forever, so if the
filesystem was originally created and populated on an older kernel
(e.g. using ext3) it would not contain a record of those writes.
There is also some potential loss if the filesystem isn't unmounted
cleanly.

It definitely _can_ be used to monitor the writes to a particular
filesystem over the past 24h, which is what the original poster was
asking about.

Cheers, Andreas





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