On 30/12/16 05:50, Doug Dumitru wrote:
Mr. Goryachev,
I find it easier to look at these numbers in terms of IOPS. You are
dealing with 17,500 IOPS vs 100 IOPS. This pretty much has to be the
"commit" from the drive. The benchmark is basically waiting for the
data to actually hit "recorded media" before the IO completes. The
"better" drive is returning this "write ACK" when the data is in RAM,
the the "worse" drive is returning this "write ACK" after the data is
somewhere much slower (probably flash). I would note that 17,500 IOPS
is a "good" but not "great" number.
So what would you consider a great number? I guess in practice the
environment isn't really that massive, it shouldn't really *need* great
numbers, but it seems no matter how hard I try to "over-architect", it
is still not performing to end user expectation.
Doing commit writes to Flash is expensive. Not only do you have to
wait for the flash, but you have to update the mapping tables to get
to the data. Flash also does not typically allow 4K updates (even
given the erase rules), so your 4K sync update probably has to update
a 16K "page" is probably causing a lot of flash wear. Maybe as much
as 10:1 write amplification. Maybe more.
Wear doesn't seem to have been a problem so far.
9 Power_On_Hours_and_Msec 0x0032 000 000 000 Old_age
Always - 914339h+46m+34.180s
This is obviously wrong, I haven't had the drive for >100 years, but it
is at least almost 4 years old (early 2013 I suspect).
233 Media_Wearout_Indicator 0x0032 095 095 000 Old_age
Always - 0
This is the worst drive out of the whole array, the best is 99, but
either way it suggests these drives could easily last >10 years, which
would be well and truly longer than their expected/useful life (based on
capacity).
241 Host_Writes_32MiB 0x0032 100 100 000 Old_age
Always - 3329773
This is the drive with the highest number of writes.... Obviously most
writes are smaller than 32MB, so I'm not entirely sure what this means,
but I suspect we are not doing a lot of writes per day compared to the
total storage capacity...
3329773 * 32MB / 3 years / 365 days = 97308MB/day. Total capacity is
480*7 = 3360GB or approx 0.03 per drive writes per day.
I've actually asked this question before, but here again we find what
appears to be an anomaly... some drives have significantly more writes
than others, and I don't understand why in a RAID5 array this would be
the case, I would have expected the writes to be split approx equally
across all drives...
225 Host_Writes_32MiB 0x0032 100 100 000 Old_age
Always - 1501762
225 Host_Writes_32MiB 0x0032 100 100 000 Old_age
Always - 1712480
225 Host_Writes_32MiB 0x0032 100 100 000 Old_age
Always - 1684811
225 Host_Writes_32MiB 0x0032 100 100 000 Old_age
Always - 1781849
225 Host_Writes_32MiB 0x0032 100 100 000 Old_age
Always - 2282764
225 Host_Writes_32MiB 0x0032 100 100 000 Old_age
Always - 2269957
225 Host_Writes_32MiB 0x0032 100 100 000 Old_age
Always - 2154155
225 Host_Writes_32MiB 0x0032 100 100 000 Old_age
Always - 2163563
225 Host_Writes_32MiB 0x0032 100 100 000 Old_age
Always - 3329774
It's hard to calculate whether some drives were replaced or similar due
to the nonsense power on hours values.... but generally all these drives
were purchased at the same time, and so should have been used mostly
equally.
In the case of RAID-5 in a "traditional" Linux deployment, and
especially with DRBD protecting you on another node, you are probably
fine without having every last ACK "perfect". After all, if you power
fail the primary node, the secondary will become the primary, and any
"tail writes" that are missing will get re-sync'd by DRBD's hash
checks. And because the amount of data being re-synced is small, it
will happen very quickly and you might not even notice it.
Right, and at this stage I'm not even looking at data integrity, I'm
only examining "performance". In fact, it would be within the
"acceptable" parameters" to lose some data under a "disaster" scenario
(where disaster means losing both primary and secondary in an unclean
shutdown). Of course, I wouldn't design the system to do that, but it
isn't a strict requirement, as long as "normal" processes mean no data
loss/corruption, and any drive should (eventually) write all the data it
has told you it will.
Back to performance, you should also consider what your array is doing
to you. You are running an 8 drive raid-5 array. This will limit
performance even more because every write becomes 2 sync writes, plus
6 reads. With q=1 latencies, if you run this test on the array with
"good" drives, you should probably get about 15K IOPS max, but it
might be a bit worse as the read and write latencies add for each OP.
Right, and one thing I've considered was moving to RAID10 to avoid this,
but even RAID10 means 2 writes. Assuming reads are relatively quick,
than that should reduce the impact of the RAID5 as well. At this stage,
converting to RAID10 is still something I'm holding up my sleeve as a
last resort (due to the additional wasted capacity).
Note that my tests are all on single drives, not the array. I can't
afford to be doing testing on the full array due to the destructive
nature, and also it is almost impossible to get a quiet moment where the
tests wouldn't be affected by workload.
I tried your test on our "in house" "server-side FTL" mapping layer on
8 drives raid-5. This is an E5-1650 v3 w/ an LSI 3008 SAS controller
and 8 Samsung 256GB 850 Pro SSDs. The array is "new" so it will slow
down somewhat as it fills. 439K IOPS is actually quite a bit under
the array's bandwidth, but at q=1, you end up benchmarking the
benchmark program. (at q=10, the array saturates the drives linear
performance at about 900K IOPS or 3518 MB/sec).
root@ubuntu-16-24-2:/usr/local/ess# fio --filename=/dev/mapper/ess-md0
--direct=1 --sync=1 --rw=write --bs=4k --iodepth=1 --runtime=60
--time_based --group_reporting --name=ess-raid5 --numjobs=1
Would it be possible for you to run the test on a single drive directly
instead?
Run status group 0 (all jobs):
WRITE: io=102653MB, aggrb=1710.9MB/s, minb=1710.9MB/s,
maxb=1710.9MB/s, mint=60001msec, maxt=60001msec
I might be looking at the wrong value, but you are getting 1711MB/s out
of an 8 drive array, I got a max of 99MB/s on a single drive, even if I
multiply that by 7 (8 drives - 1 redundancy), it's still less than half.
I'd be pretty keen to see your single drive results. Also whether those
results will change when using the 800GB model.
Thank you for your advice, I'll see whether I can find a way to purchase
one of the samsung drives for testing/evaluation, then seem to be a
similar price to the Intel S3510 that I was looking at.
Regards,
Adam
On Wed, Dec 28, 2016 at 6:14 PM, Adam Goryachev
<mailinglists@xxxxxxxxxxxxxxxxxxxxxx> wrote:
Apologies for my prematurely sent email (if it gets through), this one is
complete...
Hi all,
I've spent a number of years trying to build up a nice RAID array for my
SAN, but I seem to be slowly solving one bottle neck only to find another
one. Right now, I've identified the underlying SSD's as being a major factor
in that performance issue.
I started with 5 x 480GB Intel 520s SSD's in a RAID5 array, and this
performed really well.
I added 1 x 480GB 530s SSD
I added 2 x 480GB 530s SSD
I now found out that performance of a 520s SSD is around 180 times faster
than a 530s SSD. I had to run many tests, but eventually I found the right
things to test for (which matched my real life results), and the numbers
were nothing short of crazy.
Running each test 5 times and average the results...
520s: 70MB/s
530s: 0.4MB/s
OK, so before I could remove and test the 520s, I removed/tested one of the
530s and saw the horrible performance, so I bought and tested a 540s and
found:
540s: 6.7MB/s
So, around 20 times better than the 530, so I replaced all the drives with
the 540, but I still have worse performance than the original 5 x 520s
array.
Working with Intel, they swapped a 530s drive for a DC3510, and I then found
the DC3510 was awesome:
DC3510: 99MB/s
Except, a few weeks back when I placed the order, I was told there is no
longer any stock of this drive, (I wanted 16 x 800GB model), and that the
replacement model is the DC3520. So I figure I won't just blindly buy the
DC3520 assuming it's performance will be similar to the previous model, so I
buy 4 x 480GB DC3520 and start testing.
DC3520: 37MB/s
So, 1/3rd of a DC3510, but still better than the current live 540s drives,
but also still half the original 520s drives.
Summary:
520s: 70217kB/s
530s: 391kB/s
540s: 6712kB/s
330s: 24kB/s
DC3510: 99313kB/s
DC3520: 37051kB/s
WD2TBCB: 475kB/s
* For comparison, I had a older Western Digital Black 2TB spare, and ran the
same test on it. Got a better result than some of the SSD's which was really
surprising, but it's certainly not an option.
FYI, the test I'm running is this:
fio --filename=/dev/sdb --direct=1 --sync=1 --rw=write --bs=4k --iodepth=1
--runtime=60 --time_based --group_reporting --name=IntelDC3510_4kj1
--numjobs=1
All drives were tested on the same machine/SATA3 port (basic intel desktop
motherboard), with nothing on the drive (no fs, no partition, nothing trying
to access it, etc..).
In reality, I tested iodepth from 1..10, but in my use case, the iodepth=1
matches is the relevant number. At higher iodepth, we see performance on all
the drives improve, if interested, I can provide a full set of my
results/analysis.
So, my actual question... Can you suggest or have you tested any Intel (or
other brand) SSD which has good performance (similar to the DC3510 or the
520s)? (I can't buy and test every single variant out there, my budget
doesn't go anywhere close to that).
It needs to be SATA, since I don't have enough PCIe slots to get the needed
capacity (nor enough budget). I need around 8 x drives with around 6TB
capacity in RAID5.
FYI, my storage stack is like this:
8 x SSD's
mdadm - RAID5
LVM
DRBD
iSCSI
From my understanding, it is DRBD that makes everything a iodepth=1 issue.
It is possible to reach iodepth=2 if I have 2 x VM's both doing a lot of IO
at the same time, but it usually a single VM performance that is too
limited.
Regards,
Adam
--
Adam Goryachev Website Managers www.websitemanagers.com.au
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