J. P. Gilliver (John) wrote:
In message , Yousuf Khan
writes:
On 24/03/2012 6:28 PM, J. P. Gilliver (John) wrote:
When it is said that they "are a good host" or "not so good", is that in
terms of performance, or longevity? I'd have thought that in terms of
performance, even if non-optimal, having almost any file on an SSD would
be better; but I could also believe that certain much-written files
would significantly shorted the life of the SSD, especially if not
optimised (is that what this "Trim" thing is about?).

Yeah, longevity is my major concern here too, so should I avoid
putting anything that has too much writing happening to it? As for
Trim, it's a command that tells the SSD that a sector is no longer in
use, so it can go in and erase that area during idle moments in the
background.

Yousuf Khan

I'm not understanding what you mean by "erase" here. Are SSDs different
in some way, i. e. aren't bits erased anyway when overwritten?

In flash, a block consists of perhaps 64 pages.

Erasure (i.e. preparing for write) is done on blocks. The entire
block is erased. But you can write new data into a freshly erased
block, in units of pages. A page might be 2KB, a block 128KB, so
there are 64 pages per block. Page size and block size change
as the flash gets smaller geometry cells and higher chip densities.

The drive works most efficiently, if you need to write block-sized
things. Writing "small things", smaller than the block size, leads
to "write amplification". Write amplification refers to needing to
erase and rewrite major portions of a block. This article has some
nice pictures, showing how the SSD controller works behind the
scenes, to get the best usage from the flash.

http://en.wikipedia.org/wiki/Write_amplification

Paul