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.
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 <tdm27@xxxxxxxxx> - 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)
