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On 12/03/14 15:10, Dimitri Maziuk wrote:
> On 12/03/2014 01:16 PM, Phil Stracchino wrote:
>> On 12/03/14 13:20, Dimitri Maziuk wrote:
> 
>>> "decent-size SSD device" -- don't make a 2.5TB spool partition 
>>> because there'll be no write leveling.
>> 
>> Why do you assume that there would be no write leveling across a
>> 2.5TB spool partition?
> 
> IRL there probably would be because you're probably not going to
> write out an entire 2.5TB LTO-6 tape at once. If you were spooling
> an entire tape, then you'd be writing an entire 2.5TB spool
> partition. Then the question is would the drive's firmware spread
> the writes over OS partition boundaries...

The drive's firmware can't spread writes across multiple devices, of
course.  You probably don't have multiple partitions on an SSD if one
of them is a 2.5TB partition ...   but let's assume you do, in fact,
have a single 10TB SSD with a 2.5TB spool partition on it.  That
partition is well above the level at which write leveling occurs, and
the SSD will level writes across the entire device without paying any
attention to what partition a particular piece of data is in.  That's
what write leveling *is*.  It makes sure all blocks of the device get
exercised as evenly as possible regardless of the pattern of data
writes at the filesystem level.

Suppose, on the other hand, that your 2.5TB spool partition is a
stripe over 5 x 512MB SSDs.  Because it's striped, spooled data is
going to be divided pretty evenly across the five devices ... each of
which is going to write-level its own writes so as to distribute write
cycles evenly across the entire media.

So, really, it's not an issue.  Either you have write leveling, or you
don't; and all modern devices do; and if you have write leveling, then
the high-level arrangement of files and filesystems and partitions
really bears very little relevance to where on the actual array of
memory cells that data physically gets written any specific time.  The
drive's firmware keeps track of that so that you don't have to.


- -- 
  Phil Stracchino
  Babylon Communications
  phils AT caerllewys DOT net
  phil AT co.ordinate DOT org
  Landline: 603.293.8485
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