[ZFS] Using SSD with partitions

Bob Friesenhahn bfriesen at simple.dallas.tx.us
Sun Oct 16 21:13:28 UTC 2011


On Sun, 16 Oct 2011, Jeremy Chadwick wrote:
>
> 2) I would like an explanation as to what "SSDs are more likely than an
> MHDD to lose data on a power outage" means exactly (on a technical
> level, not something vague) and from where you got this interpretation.

The reason is that normal operation of the SSD will move and/or 
rewrite existing data, which is also likely to be much older than the 
data currently being written.  Common reasons are wear leveling, 
garbage collection (compacting) and because the block written is not 
identically sized and aligned with the SSDs native underlying blocks.

While data is being re-written, moved, or copied, a copy resides in 
RAM.  A SSD which is more defensive about avoiding corrupting old data 
is also likely to be slower to synchronously write.

There are certainly algorithms (e.g. as used by zfs) which can help an 
SSD avoid issues.

Bob
-- 
Bob Friesenhahn
bfriesen at simple.dallas.tx.us, http://www.simplesystems.org/users/bfriesen/
GraphicsMagick Maintainer,    http://www.GraphicsMagick.org/


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