Re: Is TRIM/DISCARD going to be a performance problem?

[Ric Wheeler <rwheeler@redhat.com>]

On 05/11/2009 10:27 AM, Theodore Tso wrote:
> On Mon, May 11, 2009 at 09:10:15AM -0400, Greg Freemyer wrote:
>> That implies that the SSD folks are not treating erase blocks as a
>> contiguous group of sectors.
>
> Correct.
>
>> For some reason, I thought their was
>> only one mapping per erase block and within the erase block the
>> sectors were contiguous..
>
> No, if you try to treat erase blocks as a contiguous group of
> sectors, you'll have terrible write amplification problems (leading to
> premature death of the SSD) and terrible small random write
> performance.  Flash devices optimized for digital cameras might have
> done that, but for SSD's, this will result in catastrophically bad
> performance, and very limited lifespan.  As I said, I expect these
> SSD's to be weeded out of the market very shortly.
>
> For any sane implementation of an SSD, the mapping will be on a per
> LBA basis, not on a per-erase block basis.
>
>> More realistic is to figure out a way to make it deterministic at
>> least for the short term (by writing data to all the trimmed blocks?),
>> then reshaping, then having a tool to scan the filesystem and re-issue
>> all the trim commands.
>
> Writing data to all of the trimmed block?  Um, no.  That would be a
> diaster, since it accelerates the wear and tear of the SSD.  The whole
> *point* of the TRIM command is to avoid needing to do that.
>
> The whole worry about determinism is highly overrated.  If the
> filesystem doesn't need a block, then it doesn't need it.  What you
> read after you send a TRIM command, whether it is the old data because
> the device applied some kind of rounding, or random data, or all
> zero's, won't matter to the filesystem.  Why should the filesystem
> care?  I know I certainly don't....
>
> 					- Ted


The key is not at the FS layer - this is an issue for people who RAID these 
beasts together and want to actually check that the bits are what they should be 
(say doing a checksum validity check for a stripe).

ric