Re: Software RAID and TRIM

On 29/06/2011 12:33, Tom De Mulder wrote:

On 28/06/11, David Brown wrote:

quoted

However, AFAIUI, you are wrong about TRIM being essential for the
continued high performance of SSDs. As long as your SSDs have some
over-provisioning (or you only partition something like 90% of the
drive), and it's got good garbage collection, then TRIM will have
minimal effect.

While you are mostly correct, over time even consumer SSDs will end up
in this state.

I don't quite follow you here - what state will consumer SSDs end up in?

Maybe I should have specified--my particular aim is to try and use
(fairly high-end) consumer SSDs for "enterprise" server applications,
hence the research into RAID. Most hardware RAID controllers that I know
of don't pass on the TRIM command (for various reasons), so I was hoping
to have more luck with software RAID.

Now you know /why/ hardware RAID controllers don't implement TRIM!


Have you tried any real-world benchmarking with realistic loads with a single SSD, ext4, and TRIM on and off?  Almost every article I've seen on the subject is using very synthetic benchmarks, almost always on windows, few are done with current garbage-collecting SSDs.  It seems to be accepted wisdom from the early days of SSDs that TRIM makes a big difference - and few people challenge that with real numbers or real thought, even though the internal structure of the flash has changed dramatically (transparent compression, for example, gives a completely different effect).

Of course, if you /do/ try it yourself and can show clear figures, then I'm willing to change my mind :-)  If I had a spare SSD, I'd do the testing myself.