Re: Software RAID and TRIM

nice,
anyone know if freebsd or netbsd or other o.s. have this (raid trim)
to do some benchmarks without losing our time developing?

2011/6/29 Tom De Mulder [off-list ref]:

On Wed, 29 Jun 2011, David Brown wrote:

quoted

quoted

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.

quoted

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.


Best,

--
Tom De Mulder [off-list ref] - 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)

-- 
Roberto Spadim
Spadim Technology / SPAEmpresarial
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