[kwlug-disc] SSDs and TRIM

Lori Paniak ldpaniak at fourpisolutions.com
Thu Apr 29 22:18:19 EDT 2010

On Thu, 2010-04-29 at 19:01 -0400, John Van Ostrand wrote:
> ----- "Lori Paniak" <ldpaniak at fourpisolutions.com> wrote:
> > I'd say get a disk with good idle GC and let each disk take care of
> > itself.  Don't bother worrying about the OS trying to operate TRIM
> > through LVM/RAID/USB/???
> How does it know which sectors are garbage?

I'm certain you know more about the details of this topic than I do but,
from the document I quoted in the post:

> When it is time to consolidate blocks to free up space, the SSD must copy all of the data it considers valid to a new
> block before it can erase the current block. Without trim, the SSD doesn’t know a page is invalid unless the LBA
> associated with it has been rewritten.
> Trim is simply the function of the operating system telling the drive that a page is no longer valid.

I take this to mean that garbage collection autonomous of the OS (trim)
will only occur on blocks for which the associated addressing info has
been re-written.  I guess the disk knows about that and the OS can be
depended upon to (occasionally) rewrite addressing info?  Trim allows
the SSD GC to operate precisely on finer disk (filesystem?) structures
for a more efficient and complete recovery of unused resources.

What exactly is the relationship between a "page" and a "block"?

Does that make any sense?

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