Hello,
apparently I have found an issue with btrfs: performance reduces with
nodatasum and multi-device "raid0" or "single".
I was testing with a series of 8 LIO ramdisks, with btrfs on those in
multi-device single mode, and writing zeroes on the filesystem with 16
dd in parallel.
Performance decreases significantly if the filesystem is mounted with
nodatasum, or with nodatacow which implies nodatasum.
CPU occupation also reduces, together with speed, as seen with htop.
At first I thought it was my problem, but then I saw this web page
http://www.linux-mag.com/id/7308/3/
which also reports reduced performance with nodatasum and multi-device
raid0 or single, e.g. see these two lines:
Btrfs
two disks,
single
standard
50.144 50.264 126.984 131.130
Btrfs
two disks,
single
nodatacow,
nodatasum
43.834 47.603 131.612 131.470
similarly with raid0
even more with compression:
Btrfs
two disks,
raid0
-o compress
70.234 69.048 130.852 129928
Btrfs
two disks,
raid0
-o compress
nodatacow,
nodatasum
48.762 48.831 130.812 130.202
If you go higher with the performances, such as with ramdisks, in the
GB/sec range, it reduces more than that. I have noticed upto 50% reduction.
It would be important to fix this problem for high-performance usages of
btrfs.
Best regards
BM
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