[gentoo-amd64] Re: Is my RAID performance bad possibly due to starting sector value?

[Duncan <1i5t5.duncan@cox.net>]

Duncan posted on Fri, 28 Jun 2013 03:36:10 +0000 as excerpted:

> So now I guess I send this and do some more testing of real device, now
> that you've provoked my curiosity and I have the 50 GB (mostly)
> pseudorandom file sitting in tmpfs already.  Maybe I'll post those
> results later.

Well, I decided to use something rather smaller, both because I wanted to 
run it against my much smaller btrfs partitions on the ssd, and because 
the big file was taking too long for the benchmarks I wanted to do in the 
time I wanted to do them.

I settled on a 4 GiB file.  Speeds are power-of-10-based since that's 
what dd reports, unless otherwise stated.  Sizes are power-of-2-based 
unless otherwise stated.  This was filesystem-layer-based, not direct to 
device, and single I/O task, plus whatever the system might have had 
going on in the background.

Also note that after reading the dd manpage, I added the conv=fsync 
parameter, hoping that gave me more accurate speed ratings due to the 
reducing the write-caching.

SSD speeds, dual Corsair Neutron n256gp3 SATA-600 ssds, running btrfs 
raid1 data and metadata:

To SSD: peak was upper 250s MB/s over a wide blocksize range of 1 MiB to 
1GiB.  I believe the btrfs checksumming might lower speeds here somewhat, 
as it's quite lower than the rated 450 MB/s sequential write speed.  

From SSD: peak was lower 480s MB/s, blocksize 32 KiB to 512 KiB (smaller  
blocksize range but much smaller block than I expected).  This is MUCH 
better, far closer to the 540 MB/s ratings.

To/from SSD: At around 220 MB/s, peak was somewhat lower than write-only 
peak, as might be expected.  Best-case blocksize range seemed to be 256 
KiB to 2 MiB.

So, best mixed-access case would seem to be a blocksize near 1 MiB.

I did a few timed cps also, then did the math to confirm the dd numbers.  
They were close enough.

Spinning rust speeds, single Seagate st9500424as, 7200rpm 2.5" 16MB 
buffer SATA-300 disk drive, reiserfs.  Tests were done on a partition 
located roughly 40% thru the drive.  I didn't test this one as closely 
and didn't do rust-to-rust tests at all, but:

To rust: upper 70s MB/s, blocksize didn't seem to matter much.

From rust: upper 90s MB/s, blocksize upto 4 MiB.

-- 
Duncan - List replies preferred.   No HTML msgs.
"Every nonfree program has a lord, a master --
and if you use the program, he is your master."  Richard Stallman