Hi folks,
In making an attempt to be ready for this month's meeting (which might
need to change date, as my son's Christmas Carols is on the 18th), I
have been playing with some different ZIL devices.
Thus far, I'm not nearly as happy as I was last time I played with this
- and whilst I'm betting that it's my actual system that's the problem,
I thought I'd throw it out there to see if anyone had any thoughts
before I *really* start sinking time into this...
Bottom line is that I'm getting crappy woeful performance from the SSD.
Sure, it's substantially better in terms of random IOPS than an HDD, but
it's nowhere where what I had previously come to expect.
dd'ing sequentially to /dev/rdsk/cxtxdxs0 with 4K blocks gives me only
about 1000 IOPS, which is about 90x less than the quoted maximum - and
is hardly setting the world on fire.
The SSD is a Samsung 840 pro - and I was expecting it to be better than
my trusty old Intel 520 series jobbie, which is presently anywhere up to
5X faster than the sammy, which is, according to the box, supposed to be
able to do 90,000 random write IOPS and about 500 odd MB/s. It's plugged
into a Gigabyte motherboard, with Sata3 controllers - and is training up
at Sata 3 signalling speed.
Things to know:
- Maxphys:
> maxphys::print -td
int 0t131072
- emulation-rmw isn't set in sd.conf (I'll check it out at some point
- and explicitly set it, but it shouldn't be getting in my way if I'm
doing 4K aligned block writes.)
- Slice 0, which is what I'm using for my testing is indeed 4K block
aligned:
Part Tag Flag First Sector Size Last Sector
0 usr wm 256 223.56GB 468845710
So - 256 * 512 bytes == 131072 ---->
131072 / 4096 == 32 ----> So we are 4K aligned.
- When using the samsung, I seem to get *fewer* IOPS when I dd
directly to the slice than ZFS gets when using it as a ZIL. I really
don't understand this one... yet. ;) (The Intel one gives me more IOPS
using dd than ZIL, which more like what I expected.)
- I'm not seeing a bazillion errors in an iostat -En. I am seeing some
illegal requests - but a reasonably low number and they aren't
skyrocketing when I blast the drives. (I really should look into why I'm
getting illegal requests to the drives though - I'm seeing some issues
on all my drives... More than I'd like)
- I have tried jacking with things to get the CPU's to spend more time
in their full power c-state. It makes about a 10% difference, so not the
multi-thousand % increase I'm looking for. ;)
Should anyone have any thoughts, I'm open to them.
I'll be poking more - but thought it would be worth an ask.
Cheers,
Nathan.
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