Author Topic: To Cache drive or not to Cache drive?  (Read 81213 times)

Offline jowi

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Re: To Cache drive or not to Cache drive?
« Reply #240 on: July 28, 2012, 01:08:20 AM »
Ok, here's an interesting case regarding cache.

If i'm correct, the basic principle of using a cache disk is that it allows unraid to use 'cached' user shares, so that if you copy data to a user share, it is not copied directly to the array/disk, but instead is copied to the (unprotected) cache disk, which speeds things up. The data still seems to be in the user share, but in stead it's on the cache disk, and the mover process will distribute all 'cached' data over the array at night. So far so good.

But now... my cache disk is an 128GB SSD, and i have installed sabnzb on it. Sabnzb's completed results are also saved on the cache disk (ssd) in a folder 'download'. Once complete, i want to move my downloaded files, let's say it's an episode of a series, to the appropriate user share, in this case '/tvseries/blah/season1/'.

This user share is offcourse, 'cached'. So if i copy data to it, it is not copied to the array, but to my SSD... and this is where things get interesting, since the data i'm copying, allready IS on the SSD...

So, unRAID is now copying a file from a location on the SSD to another location on the same SSD at 50MB/s... which is pretty inefficient and basically rendering the whole principle of caching, useless... this way it still can take hours to copy a few HDTV seasons from SSD to SSD...

My workaround in this case is just to 'fake' unRAID's cached user share by creating the /tvseries/blah/season1 folder MYSELF on the SSD, and MOVE (ctrlX/ctrlV) gigabytes at once in a split second, bypassing the inefficient copy step... and the result is the same, and later that night, the mover process will still distribute it over the array...

(to be hounest, since my copy speed to a cached, ssd user share is just 50MB/s, i ALWAYS use the above workaround, these sort of slow speeds renders the use of a fast, transparent ssd cache, pretty much useless)

So, basically, unRAID's cached user share should be made aware if the data being copied to it, is allready on the same fysical location and act accordingly.
« Last Edit: July 28, 2012, 01:18:36 AM by jowi »
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Offline Influencer

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Re: To Cache drive or not to Cache drive?
« Reply #241 on: July 28, 2012, 07:01:42 AM »
When you are moving the files from Sab, are you doing it via telnet/ssh or windows/samba.

From your previous screens it appears your cpu is your bottleneck. When writing to cache directly smbd can use the entire CPU, netting you higher speeds. When copied to user share, shfs kicks in, stealing some CPU cycles and slowing things down.
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Offline jowi

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Re: To Cache drive or not to Cache drive?
« Reply #242 on: July 28, 2012, 07:17:29 AM »
When you are moving the files from Sab, are you doing it via telnet/ssh or windows/samba.
Usually from windows/samba, but same speeds occur when doing internal copying on telnet.

From your previous screens it appears your cpu is your bottleneck. When writing to cache directly smbd can use the entire CPU, netting you higher speeds. When copied to user share, shfs kicks in, stealing some CPU cycles and slowing things down.
One would say an 1.8GHz Atom CPU would be sufficient to handle some basic file copying?
[36TB UNRAID SERVER v5.0.4]
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Running : Dynamix GUI • APCUpsd • Sickbeard • Sabnzbd • DropBox • Subversion • SqueezeServer

Offline BRiT

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Re: To Cache drive or not to Cache drive?
« Reply #243 on: July 28, 2012, 08:17:03 AM »
Not entirely. It's quite a slow cpu, also some chipsets for the atom have slower disk performance than the Intel i3/i5/i7 lines. :(
« Last Edit: July 28, 2012, 08:19:17 AM by BRiT »

Offline jowi

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Re: To Cache drive or not to Cache drive?
« Reply #244 on: July 28, 2012, 08:40:02 AM »
Mmm...  I've been lurking a lot on the forum and read great things about this board in combination wit unRAID, and now it turns out it's basically too slow for what i want, or my expectations were too high i suppose... either way, maybe i should go the Intel i3 route after all, with a nice X9SCM-F board...
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Running : Dynamix GUI • APCUpsd • Sickbeard • Sabnzbd • DropBox • Subversion • SqueezeServer

Offline Influencer

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Re: To Cache drive or not to Cache drive?
« Reply #245 on: July 28, 2012, 10:26:06 PM »
It's no longer basic file copying when copying to a user share, that's why its becoming a bottleneck.

Writing directly to cache shfs isn't used, so it uses no cpu. Writing to the user share, it is writing to the same location but now shfs is looking at the data, where its destination is, the settings for that share(cache or not, split level, fill settings, amount free on disks in that share) to figure out where its going to end up. This spike in processing eats up valuable cycles, something that probably would never be noticeable but since your using a fast SSD, you've moved the writing bottleneck from the drive to the cpu.

I may be wrong, I'm no expert by any means, but its my interpretation.
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Offline jowi

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Re: To Cache drive or not to Cache drive?
« Reply #246 on: July 29, 2012, 12:37:13 AM »
I'm having the same 'speeds' when replacing the SSD with an old 500GB harddisk... so it's not the SSD that is pushing the limits of the cpu, i guess it's the whole user share principle that on it's own is pushing the Atom board to it's limits. I saw another topic where someone experiences 20% slower speed when copying to a user share compared to copying to a disk, i'm experiencing this as well. This guy also has an Atom board... looks like the Atom board is just not fast enough for utilizing unRAID to it's full potential.

Not to be blunt, but i think unRAID has to be more specific in terms of minimal hardware specifications. For newbies like myself, it's hard to find up to date info on things like this, a lot of info on the wiki and the Lime site is just plain old. The recommended motherboard has boards from 2007... so you start reading on the forum and find that this Supermicro Atom board is widely appreciated used, so it must be ok...
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Offline Influencer

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Re: To Cache drive or not to Cache drive?
« Reply #247 on: July 29, 2012, 05:41:36 AM »
Good point. I went with a beefier CPU out of the gate to support other add-ons, but apparently the atom isn't quite fast enough. Learned something new
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Offline Raf (B)

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Re: To Cache drive or not to Cache drive?
« Reply #248 on: July 29, 2012, 06:08:20 AM »
I'm having the same 'speeds' when replacing the SSD with an old 500GB harddisk... so it's not the SSD that is pushing the limits of the cpu, i guess it's the whole user share principle that on it's own is pushing the Atom board to it's limits.
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.

Offline jowi

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Re: To Cache drive or not to Cache drive?
« Reply #249 on: July 29, 2012, 06:50:58 AM »
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!
[36TB UNRAID SERVER v5.0.4]
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Offline Raptor

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Re: To Cache drive or not to Cache drive?
« Reply #250 on: July 29, 2012, 09:07:15 AM »
here's my result of copying data to/from cache drive
i5-4670 | Z87extreme4 | SSD 120GB/240GB | HDD: 5x 3TB | Host: Arch + KVM + SnapRAID | VM: W7U & HD7950



Offline jowi

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Re: To Cache drive or not to Cache drive?
« Reply #251 on: July 29, 2012, 09:24:27 AM »
But do you see difference in speed if you copy to the cache drive, e.g. /mnt/cache/temp or to a cached user share? That is the question. And if so, how much difference?

Those speeds you have are great by the way 8)
[36TB UNRAID SERVER v5.0.4]
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Offline Raptor

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Re: To Cache drive or not to Cache drive?
« Reply #252 on: July 29, 2012, 09:43:23 AM »
no difference
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Offline jowi

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Re: To Cache drive or not to Cache drive?
« Reply #253 on: July 29, 2012, 09:52:10 AM »
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,-...
[36TB UNRAID SERVER v5.0.4]
Supermicro X9SCM-F • IBM M1015 IT mode • Intel® CoreTM i3-2120T CPU @ 2.60GHz • 4GB DDR3 ECC • 9x 4TB Hitachi HDS724040ALE640 • 128GB Corsair Performance Pro SSD
Running : Dynamix GUI • APCUpsd • Sickbeard • Sabnzbd • DropBox • Subversion • SqueezeServer

Offline jowi

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Re: To Cache drive or not to Cache drive?
« Reply #254 on: July 29, 2012, 10:15:10 AM »
@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.
[36TB UNRAID SERVER v5.0.4]
Supermicro X9SCM-F • IBM M1015 IT mode • Intel® CoreTM i3-2120T CPU @ 2.60GHz • 4GB DDR3 ECC • 9x 4TB Hitachi HDS724040ALE640 • 128GB Corsair Performance Pro SSD
Running : Dynamix GUI • APCUpsd • Sickbeard • Sabnzbd • DropBox • Subversion • SqueezeServer