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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 ...
The disk health looks fine to me too.
FWIW, the 3 TB Seagate DM drives are known to have high failure rates with RAID. But there's no evidence of problems with your particular drives.
When the time comes to replace them (and the WD green), I recommend using NAS-purposed drives - WDC Red or Seagate Ironwolf models. I use Reds myself.
So it's not the disk health, and it's not something loading down the CPU (other than the scrub itself). Have you tried measuring NAS throughput when scrubs aren't running? For instance using NAStester on a PC? http://www.808.dk/?code-csharp-nas-performance
Yes, I have. In particular, to take the network out of the picture, I've tried to just sha1sum the files on the disk. This is a worst case, since the sha1 itself should take quite a lot of CPU. It runs much faster than the scrubs, around 1.5 GB/minute. At that rate, I would go through the full 4TB in about 1.5-2 days, which is just what I see if I run an sha1sum on every file using find. This is what I would expect to see from the scrub, or better. So the NAS has no trouble reading all the data off these disks in some reasonable amount of time.
This is totally a software bug.
a) Same hardware
b) Only the OS version changed
c) Nothing running
d) No hardware problems
e) Hardware has no problem reading and even sha1 hashing the data in a reasonable amount of time
f) defrags and rebalances work in a reasonable amount of time (~1 day)
g) Scrubs are horrifically slow (~20+ days), at least 10x slower than anything else that runs on this thing.
There's really nothing else it could be.
It would have to be a bug/performance bottleneck in BTRFS itself.
BTRFS checksums use CRC32c, and they are block-based not file based (done on 4K blocks). CRC32 should be significantly faster than SHA-1 (perhaps 60% fewer cycles). Since checksum verification is always done, your sha1sum test is actually computing both the BTRFS checksums and the SHA-1 hashes.
FWIW, my last scrub took about 31 hours on my RN526. Disk configuration is 4x6TB RAID-5, with about 8.5 TiB of data (including snapshots). Roughly 75 MB/s.
For 4.2Tb raid 1 it took me around 15hrs!
On my RN316.
aalexandrebeta wrote:
For 4.2Tb raid 1 it took me around 15hrs!
On my RN316.
That sounds about right actually.
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