To Cache drive or not to Cache drive?

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Ok, thanks. It's clear to me that unRAID has to adjust it's minimal required cpu to an i3@2GHz or better if you want to fully enjoy a gigabit connection and a cache drive without any penalties in speed. I wish i knew that upfront... now i'm thinking of replacing my brand new X7SPA-HF-D525 with an X9SCM/i3 or something, which will set me back another €300,-...

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@Raptor, just to get an indication, what sort of speed do you see when syncing or checking parity? I have slow copy speeds to user shares and cache but parity sync speed is at an average of 100MB/s. I'm curious if that is influenced as well.

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Last checked on Sunday, 2012 July 22, 14:39 (today), finding 0 errors.

* Duration: 5 hours, 49 minutes, 9 seconds. Average speed: 95,5 MB/sec


At start speed is around 125-135MB/s

Ok, that's the same on my system (and i'm on 4TB disks, will take 9 hours, but same speed), so that is not so much affected by a 'slower' cpu, but probably has more to do with bandwidth and pci card/structure and overall architecture. Good to know.

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The parity calculations shouldn't have much of a hit, it is fairly simple calculations taking place so slower CPUs should be fine there. Plus, IIRC, shfs isn't needed for parity calculations since its direct disk to disk i/o.


Edit for clarification: I know you had come to the conclusion parity sync isn't affected, was giving a little science behind as to why, :)

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I've replaced my supermicro X7SPA with an X9SCM-F/i3-2120T. Speed increase is dramatic... writing to the cache disk directly or to a cached user share now will do 110MB/s. Writing to a non-cached user share is at 40MB/s.


(With the X7SPA writing to the cache disk was at 80MB/s, writing to a cached user share at 50MB/s, writing to a normal share at 35MB/s, and if i did more than one datatransfer at a time, speed would drop and the system would literally come to a standstill... )


My conclusion... the minimal hardware specs for unRAID should be increased, Lime should update their specs so people like me don't have to keep buying hardware that doesn't perform.

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Well i bought the same mainbord as Jowi, and have the same feelings.

I see also transfers to the cache with peaks of 72 Mbyte/sec, but often only 35-50 Mbyte/sec. It depends on the files. I copy Blu Ray rips, so very large files (20+ GB) and lots of smaller files often.


Also preclearing a drive at the same time has a large impact. The IO systems seems important, and this Atom bord is not up to what i expected either.

One thing is great, noiseless CPU, as no active cooler.

Will keep this mainbord and built a 2nd server with a Xeon with low thermal heat production "L" type.

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Wow, I am impressed,

Just received my Plus license key and rushed to install an old Seagate Momentus 5400.6 drive as a cache drive.

The increase of data transfer speed is really dramatic. I am writing from the cache drive to a user shares  with ~ 75-80 Mbyte/Sec...while before my peak was something like 35 Mbyte/Sec. I suppose with faster Cache HDD the speed will be even higher.

My CPU is an old Celleron G530 @ 2.40GHz and non fancy at all Mainboard Asus P8H61-M-LE/USB3 + 4gb cheapo RAM.


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@jowi: sadly the test in my environment could not be a benchmark as I am currently using an old vaio laptop with a 100mbit network interface. The speed is around 12 Mbyte/sec.

@raptor: your remark is correct. I mean the CPU is not a beast comparing to the others board members ones :)



Sent from my iPhone using Tapatalk

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@jowi: sadly the test in my environment could not be a benchmark as I am currently using an old vaio laptop with a 100mbit network interface. The speed is around 12 Mbyte/sec.

You could check it as well by copying from e.g. physical disk1 in your array, to the actual cache disk, and from disk1 to a cached user share.

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jowi, Raptor, Influencer, and everyone else who has chimed in on jowi's original questions:


Thank you for posting all of the detailed accounts of your testing and data to back it up. I apologize for not jumping in sooner, but I've found that between my business and other pursuits my time for these forums has dropped to once per week at best (but I'm trying to improve that!).  From the past few pages' detailed discussions it is clear to me that the Atom's CPU speed is slow enough to cripple a modern SSD's performance as an unRAID cache drive. This is crucial knowledge for anyone looking to build a low power yet zippy server, so I'll post this information on the first post of this thread and do my best to spread the knowledge elsewhere. I encourage you intrepid souls to take this information to the wiki in the 'hardware compatibility' section!


Also, I think Raptor has set the new cache drive write speed record here! I've also updated the first post to link to those results.

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After my first excitement of the speed the cache drive provide me with I have just noticed some very weird behavior of the transfer speeds.

Sometimes the speed is around 70 MB/s in other situations it just drops to 5-6 MB/s. I haven't changed anything in the system. Reboot doesnt solve the problem. It is not dependent also on the fact if the drives are spinning or not. It just writes with the speed it wants. The Media share is cashed. I am copying from the transmission (which is installed on the cache drive as app) to my Media user share. Probably my configuration is faulty that's why I am attaching here some screen-shots . 


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I decided to perform a smart test on my cache drive and saw some concerning Raw_Read_Error_Rate values.

After a research I understood that Raw_Read_Error_Rate on Seagates has always big value... is it right?

Probably this is the reason for the inconsistent transfer speeds?

Log attached.


EDIT: I decided to move some files across the Disks. So my speed is back to normal 70(something) MB/sec when i copy from cache disk to Disk1 or Disk2. The slaw transfer speeds remain when I copy from Cache drive to user shares. So according to my understanding this can't be a hardware problem, but rather some kind of a software issue.


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Thank you for your answer dgaschk,

I executed the command. The results are bellow:

sdd is the cache drive.

sda is WD7501AALS

and sdb is WD30EZRX

Strange that the black WD is slower than the green drive.

root@UNRAID:~# hdparm -tT /dev/[hs]d?



Timing cached reads:  4674 MB in  2.00 seconds = 2337.42 MB/sec

Timing buffered disk reads: 310 MB in  3.00 seconds = 103.23 MB/sec



Timing cached reads:  4666 MB in  2.00 seconds = 2333.00 MB/sec

Timing buffered disk reads: 364 MB in  3.01 seconds = 120.86 MB/sec



Timing cached reads:  4502 MB in  2.00 seconds = 2252.13 MB/sec

Timing buffered disk reads:  86 MB in  3.04 seconds =  28.26 MB/sec



Timing cached reads:  4532 MB in  2.00 seconds = 2265.68 MB/sec

Timing buffered disk reads: 200 MB in  3.01 seconds =  66.55 MB/sec

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  • 4 weeks later...

I have ordered a SuperMicro X9SCM-F with an I3 2120T to deploy as Unraid.

I did not plan to use a cache drive, but I can temporary allocate one and do some tests if you think the Atom CPU is the bottleneck in the process. Will be some weeks though before I have time to proper install / test.

That would be great! Much appreciated!

Sorry for the delay, but I ordered non-ECC memory and the server obviously did not boot.

Then there was a delay in the delivery of the ECC memory


Anyway, as it seems we have an identical configuration now (SuperMicro X9SCM-F, Intel Core I3 2120T, Hitachi Deskstar 7K4000, 8 GB RAM), I can only confirm the findings you already had.


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