Re: [9fans] Throwing in the Towel

[Steve Simon <steve@quintile.net>]

i  can only speak from experience, but i have had fossil and venti running on a single ssd (on a radpberry pi) for 5 years now - no rotating discs left at home.

i have mtime changes and ephemeral snapshots turned off to reduce the update rate. i chose a sandisk card, and take backups just in case…

so far so good.

-Steve

> On 29 May 2024, at 4:39 am, ori@eigenstate.org wrote:
> 
> Finally,. SSDs just die over time. Especially if they are
> not powered on and refreshing. JEDEC specs say that they
> should retain data for 1 year unplugged when stored at 30
> degrees celsius, assuming the internet isn't lying to me.
> 
> Keep backups.
> 
> Quoth Dave Eckhardt <davide+p9@cs.cmu.edu>:
>>> For the napkin calculation: On disk, the IEntry is 38Bytes.  Alas,
>>> writes occur always in (the ssd internal) blocksize.  So, essentially
>>> (assuming 4096 byte blocksize, which is quite optimistic), we have
>>> a write efficiency of less than 1 percent.
>> 
>> While I see how such a model can predict disaster, I don't think that
>> model matches how FTLs work, because it can't.
>> 
>> Many file systems (FAT, ext2/3/4) write the same logical block over
>> and over and over and over and over.  I think the default interval
>> for ext4 to synch the superblock and the journal is five seconds,
>> which if true is more than 15,000 times every *day* for a busy
>> file system (and I think lots of Linux systems are busy in that
>> sense).
>> 
>>> A good firmware in the ssd could avoid needing a new block for the
>>> write, if all bits are changed in teh same direction by the new
>>> data.
>> 
>> Again, I believe this model predicts that no regular Linux file
>> system can be used on any SSD, thus I believe this model is not
>> accurate.
>> 
>> To quote Wikipedia:
>> 
>>  https://en.wikipedia.org/wiki/Flash_memory_controller
>> 
>>> The mapping units of an FTL can differ so that LBAs are mapped
>>> block-, page- or even sub-page-based.  Depending on the usage
>>> pattern, a finer mapping granularity can significantly reduce
>>> the flash wear out and maximize the endurance of a flash based
>>> storage media.
>> 
>> Also, I feel as if this point is several assumption layers deep.
>> I think one user reported an unknown number of failures in two
>> sets of SSDs of unknown brand and model.  I don't think we know
>> that it was venti SSDs that went bad as opposed to fossil SSDs,
>> let alone knowing it was index SSDs for venti.
>> 
>>> It seems, venti in its current form is a ssd killer, if they
>>> are used for the isects.
>> 
>> I don't think this claim is yet supported well.
>> 
>> Dave Eckhardt

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