Re: Software RAID and TRIM

[David Brown <david@westcontrol.com>]

On 29/06/2011 14:55, Tom De Mulder wrote:
> On Wed, 29 Jun 2011, David Brown wrote:
>
>>> 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?
>
> Sorry, I meant to say "SSDs in typical consumer desktop machines". The
> state where writes are very slow.
>

Well, many consumer level systems use older or cheaper SSDs which don't 
have the benefit of newer garbage collection, and don't have much 
over-provisioning (you can always do that yourself by leaving some space 
unpartitioned - but "consumer" users would typically not do that).  And 
remember that for users in this class, who will probably have small SSDs 
to keep costs down, will have fairly full drives - making TRIM almost 
useless.

>> 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.
>
> I have a set of 4 Intel 510 SSDs purely for testing, and I have used
> these to simulate the kinds of workload I would expect them to
> experience in a server environment (focused mainly on database access).
> So far, those tests have focused on using single drives (ie. without
> RAID) on a variety of controllers.
>
> Once the drives get fuller (something which does happen on servers) I do
> indeed see write latencies that are in the order of several seconds (I
> saw from 1500µs to 6000µs), as the drive suddenly struggles to free
> entire blocks, where initially latency was in the single digits.
>
> I am hoping to get my hands on some Sandforce controller-based SSDs as
> well, to compare, but even they show degradation as they get fuller in
> AnandTech's tests (and those tests seem, IME, trustworthy).
>
> My current plan is to sacrifice half the capacity by partitioning, stick
> 2 of them in md RAID1 (so, without TRIM) and over the next few days to
> run benchmarks over them, to see what the end result is.
>

Well, try it and see - and let us know the results.  50% manual 
over-provisioning seems excessive, but I guess that's what you'll find 
out with the tests.

>
> Best,
>
> --
> Tom De Mulder <tdm27@cam.ac.uk> - Cambridge University Computing Service
> +44 1223 3 31843 - New Museums Site, Pembroke Street, Cambridge CB2 3QH
> -> 29/06/2011 : The Moon is Waning Crescent (18% of Full)


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