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Forum Discussion
BTRFS scrub speed is insanely slow.
So I have a RN104, with about 10TB of disk space. When I set it to do a BTRFS scrub, the speed is horrific. I'm getting something like 30GB/hour of scrub speed. At the present rate, it would take almost a month of continuous disk griding to scrub all my data. The BTRFS forums routinely show people having scrub speeds that are slow, but nothing like this. People are advised to scrub roughly monthly, which would never work if the scrub itself takes a whole month.
The thing is, it didn't used to be this bad. I previously scrubbed my data with an earlier version of the firmware (6.4.x? not sure, don't remember), and it performance was fine. It would finish in a day or two, which is totally reasonable. Somehow, recent firmware totally broke this functionality.
Defrag and balance are still about the same, completing in 1-2 days, but scrub has just putrid performance.
Also, I'll point out that I can manually scrub the whole thing, by simply reading every byte of every file using something like "find ./ -exec sha1sum {} \;", run from a recent snapshot. This should be doing (almost) everything the scrub is doing, but it runs at approximately 20-100x the speed, finishing in a day or less.
There is a serious bug here, have others seen it?
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Replying to my own message, it's about 4GB/hour, not 30GB/hour. I let it run for roughly 3 days (~75 hours), and it finished a bit over 300 gb total.
The last one I did took about 30 hours - that was on an RN526 with 4x6TB installed. That was using 6.6.1 beta.
That seems like a roughly plausible speed for this to be.
So, given that i have half as much data as you do, but my scrub is projected to take roughly 2,000 hours, is it really plausible that the CPU in the RN104 is 2,000 / 15 ~ 130x slower than the one in the RN526? Doesn't seem right to me.
There's definitely a bug here.
When you schedule/run SCRUB from the UI, it runs both a scrub on the RAID as well as a btrfs scrub. 2x the load.
RN100 series devices are single core, low powered CPU devices. They take quite some time to accomplish tasks compared to other units.
Thank you to educate us!! I appriciate kohdee's remarks. Could you eplainhow scrub process is running?
All true, but this doesn't come close to explaining the phenomenon.
a) It worked well in the past (ran in 1-2 days)
b) I can read and verify all the data on the disk using commandline tools on the NAS itself in 1-2 days.
So the issue isn't related to CPU speed as that hasn't changed, and in any case would slow down other operations on the NAS as well. Also, it can explain maybe a 2x difference in speed. If the difference was only 2x, I would never have even mentioned it, but the difference is more like 20x, and it came around once an updated version of the OS was installed.
It's definitely a software bug that needs to be fixed.
Hi joey123,
Your WD disk's Load Cycle Count has been 1,462,660. Actually Western Digital rates their Green drives for 300,000 cycles. Please check this page: S.M.A.R.T. So please replace this disk. WD Green drives are designed to unload heads often to conserve power. The LCC would be increased very fast when using in storage. We don't advise to use all WD Green drives in our NAS because of this.
And I saw you enabled the data checksum(on Volumes page). Actually we disable this option for all arm based NAS boxes by default because it would affect the performance.
Thanks,
-Leia
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