Re: Software RAID and TRIM

[David Brown <david.brown@hesbynett.no>]

On 28/06/11 17:31, Tom De Mulder wrote:
> Hi,
>
>
> I'm investigating SSD performance on Linux, in particular for RAID devices.
>
> As I understand it—and please correct me if I'm wrong—currently software
> RAID does not pass through TRIM to the underlying devices. TRIM is
> essential for the continued high performance of SSDs, which otherwise
> degrade over time.
>
> I don't think there would be any harm in this command being passed
> through to underlying devices if they don't support it (they would just
> ignore it), and if they do it would make high-performance software RAID
> of SSDs a possibility.
>
>
> Is this something that's in the works?
>
>

I don't think you are wrong about software raid not passing TRIM down to 
the device (IIRC, it /can/ be passed down through LVM raid setups, but 
they are slower and less flexible than md raid).

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.

TRIM only makes a big difference in benchmarks which fill up most of a 
disk, then erase the files, then start writing them again, and even then 
it is mainly with older flash controllers.

I think other SSD-optimisations, such as those in BTRFS, are much more 
important.  These include bypassing or disabling code that is aimed at 
optimising disk access and minimising head movement - such code is of 
great benefit with hard disks, but helps little and adds latency on SSD 
systems.

(I haven't done any benchmarks to justify this opinion, nor have I 
direct links - it's based on my understanding of TRIM and how SSDs work, 
and how SSD controllers have changed between early devices and current 
ones.)

--
To unsubscribe from this list: send the line "unsubscribe linux-raid" in
the body of a message to majordomo@vger.kernel.org
More majordomo info at  http://vger.kernel.org/majordomo-info.html