Hi folks, (Long time no post...)
Only starting to get into this one, so apologies if I'm light on detail,
I have a shiny SSD I'm using to help make some VirtualBox stuff I'm
doing go fast.
I have a 240GB Intel 520 series jobbie. Nice.
I chopped into a few slices - p0 (partition table), p1 128GB, p2 60gb.
As part of my work, I have used it both as a RAW device (cxtxdxp1) and
wrapped partition 1 with a virtualbox created VMDK linkage, and it works
like a champ. :) Very happy with that.
I then tried creating a new zpool using partition 2 of the disk (zpool
create c2d0p2) and then carved a zvol out of that (30GB), and wrapped
*that* in a vmdk.
Still works OK and speed is good(ish) - but there are a couple of things
in particular that disturb me:
- Sync writes are pretty slow - only about 1/10th of what I thought I
might get (about 15MB/s). ASync writes are fast - up to 150MB/s or more.
- More worringly, it seems that writes are amplified by 2X in that if
I write 100MB at the guest level, the underlying bare metal ZFS writes
200M, as observed by iostat. This doesn't happen on the VM's that are
using RAW slices.
Anyone have any thoughts on what might be happening here?
I can appreciate that if everything comes through as a sync write, it
goes to the ZIL first, then to it's final resting place - but it seems a
little over the top that it really is double.
I have also had a play with sync=, primarycache settings and a few other
things but it doesn't seem to change the behavious
Again - I'm looking for thoughts here - as I have only really just
started looking into this. Should I happen across anything interesting,
I'll followup this post.
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