To Cache drive or not to Cache drive?

[Alexandro]

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 . 

[Alexandro]

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.

[dgaschk]

Enter "hdparm -tT /dev/[hs]d?" to speed test the drives. The flash will be the slow one.

[Alexandro]

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?

/dev/sda:
 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

/dev/sdb:
 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

/dev/sdc:
 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

/dev/sdd:
 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

[Raf (B)]

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.

[dikkiedirk]

If I were to use a Samsung 830 series SSD as cache and SABNZB was installed on it using the SSD for downloading, verifying and repairing. Would the SSD then wear out soon and die quickly? Would the SSD also become slow overtime because there is no TRIM support in unraid?

[Harpz]

I currently have a 1T WD Green EARS drive as my cache drive as i had it laying about the most i ever see is 24-48 (more 48)MB/s.

Was looking at replacing it with a 320G Western Digital Black. would i see a marked improvement in the transfer speed to the server?

Also as moneys a bit tight at the moment been looking around on eBay and came across

http://www.ebay.co.uk/itm/Western-Digital-Scorpio-Black-320-GB-Internal-7200-RPM-6-35-cm-2-5-SATA-/230863509258#vi-content

and the auction states that its Seller refurbished, has anyone got any idea what that means and is it worth considering refurbished drives, Never brought one before. Experiences?

[downloadski]

I see now something strange:

syslog:

Code: [Select]

root@Tower2:/var/log# tail syslog
Oct 13 10:17:08 Tower2 shfs/user: cache disk full
Oct 13 10:17:39 Tower2 last message repeated 4 times
Oct 13 10:18:50 Tower2 last message repeated 3 times
Oct 13 10:19:57 Tower2 last message repeated 11 times
Oct 13 10:21:06 Tower2 last message repeated 9 times
Oct 13 10:22:09 Tower2 last message repeated 495 times
Oct 13 10:33:49 Tower2 last message repeated 3 times
Oct 13 10:35:27 Tower2 last message repeated 4 times
Oct 13 10:37:37 Tower2 last message repeated 17 times
Oct 13 10:38:38 Tower2 last message repeated 481 times

The copy is still running with 40-50 MByte/sec

attache image is the main view of unraid webinterface showing 460 GB free

Is this a bug ?


Update:
I found the problem, there is a minmal free space setting for the cache drive. It was set to 500 GB, so user error i guess.

[zeroryu]

I'm wondering if below stats are normal:

all my drives are WD green 3TB. i don't have cache drive setup. i'm interested in knowing if adding a cache drive will improve this stats significantly or not.

/dev/sda:
 Timing cached reads:   11988 MB in  2.00 seconds = 6003.55 MB/sec
 Timing buffered disk reads:  56 MB in  3.07 seconds =  18.25 MB/sec

/dev/sdb:
 Timing cached reads:   12502 MB in  2.00 seconds = 6260.80 MB/sec
 Timing buffered disk reads: 380 MB in  3.01 seconds = 126.14 MB/sec

/dev/sdc:
 Timing cached reads:   12616 MB in  2.00 seconds = 6318.38 MB/sec
 Timing buffered disk reads: 372 MB in  3.01 seconds = 123.60 MB/sec

/dev/sdd:
 Timing cached reads:   12444 MB in  2.00 seconds = 6231.35 MB/sec
 Timing buffered disk reads: 378 MB in  3.00 seconds = 125.86 MB/sec

/dev/sde:
 Timing cached reads:   12482 MB in  2.00 seconds = 6251.29 MB/sec
 Timing buffered disk reads: 368 MB in  3.00 seconds = 122.64 MB/sec

/dev/sdf:
 Timing cached reads:   12460 MB in  2.00 seconds = 6239.76 MB/sec
 Timing buffered disk reads: 362 MB in  3.01 seconds = 120.17 MB/sec

/dev/sdg:
 Timing cached reads:   12446 MB in  2.00 seconds = 6233.11 MB/sec
 Timing buffered disk reads: 378 MB in  3.01 seconds = 125.51 MB/sec

/dev/sdh:
 Timing cached reads:   12484 MB in  2.00 seconds = 6251.94 MB/sec
 Timing buffered disk reads: 376 MB in  3.01 seconds = 124.71 MB/sec

[Joe L.]

I'm wondering if below stats are normal:

all my drives are WD green 3TB. i don't have cache drive setup. i'm interested in knowing if adding a cache drive will improve this stats significantly or not.

The  "cache" drive improves "write" speeds to the array.  It slows down read speeds (a tiny-tiny-bit)

Write speeds are improved only because the process of writing to the protected array is delayed to a later time.  You files are NOT protected until copied to the protected array/

Read speeds are slowed because the cache drive must FIRST be scanned for any given file before scanning the protected data disks in the array.  That takes a tiny bit of time.

To answer your question, the stats on the physical drives when reading them will not change at all.  They have nothing to do with actual read and write speeds to the array.   Most disks will not be able to sustain 100MB/s read speeds.

Joe L.

[GortWillSaveUs]

Question: I am currently writing to Disk1. I am copying over 23GB of TV_Shows and I wanted it all on one drive.

I noticed that only Disk 1 and the Parity drive are active? Why not the cache drive?
Is it only active if I write data to a user share?

[dgaschk]

Question: I am currently writing to Disk1. I am copying over 23GB of TV_Shows and I wanted it all on one drive.

I noticed that only Disk 1 and the Parity drive are active? Why not the cache drive?
Is it only active if I write data to a user share?

Yes.

[GortWillSaveUs]

Thanks! I just found the "Use Cache Drive" drop down on the User Shares.
I just tested it,..and now I see the data writing to only the cache drive.

So if I write specifically to one of the Disk directly,..then no cache.
I suppose I have to reread how to setup the User shares to only use one Disk in the case of wanting to copy data to only one disk.

[Jon]

I like the cache idea and think it is a great implementation.  I just thought I'd add my 2 cents since I was bored and came across this thread

I do not use a cache drive simply because I do not implement writable user shares.  I write directly to disk shares and leave my user shares as read-only for my Media Centers.  I've been tempted to change the way I do things a few times, but I began this way nearly 5 years ago, so I guess old habits just die hard.

I have implemented one for kicks a time or two to function as a scratch drive, but I never got any real use of it and ended up removing it.  I still think it's one of the best additions in the past few years though.

[loadme]

Edit: (3/7/2011) I'll add that LimeTech has mentioned in several threads their intention to create a 'hot spare' option for unRAID, which would allow for the use of a cache drive AND a spare drive side-by-side.  Great news!

you mentioned in your main post, that there have been rumors about this feature a while ago.

any updates?