[TUHS] /dev/drum

Warner Losh imp at bsdimp.com
Wed Apr 25 00:57:22 AEST 2018

On Tue, Apr 24, 2018 at 12:22 AM, Bakul Shah <bakul at bitblocks.com> wrote:

> On Mon, 23 Apr 2018 22:59:19 -0600 Warner Losh <imp at bsdimp.com> wrote:
> > >         ...
> > >         Not_Zoned       # Zone Mode <<== this seems wrong.
> > >
> >
> > That's right. This is for BIO_ZONE stuff, which has to do with host
> managed
> > and host aware SMR drive zones. That's different than the zones you are
> > talking about.
> Ah. Thanks! Does host management of SMR zones provide better
> throughput for sequential writes? Enough to make it worht it?
> [I guess this may be something you guys may care about?]
> Haven't had a chance to work on storage stuff for ages.  [Last
> I played with Ceph was 5 years ago and at a higher level than
> disks.]

Right now, I don't think that we do anything in stock FreeBSD with the
zones. I've looked at trying to create some kind of FS that copes with the
large granularity writes blocks that host managed SMR drives would need to
do and changes we'd need to a write-in-place FS to take advantage of it.
It's possible, but it would turn UFS from a write-in-place system to a
write-in-place, but only for meta-data, and different free block allocation
methods. So far, it hasn't been enough of a win to be worth bothering with
for our application (eg, we can get 10-20% more storage, but that delta is
likely to remain constant, and the effort to make it happen is high enough
that the savings isn't there to pay for the development).

> Yes. This matches our experience where we get 1.5x better on the low LBAs
> > than the high LBAs. We're looking to 'short stroke' the drive to the
> first
> > part of it to get better performance... Toss a filesystem on top of it,
> and
> > have a more random workload and it's down to about 30% better than using
> > the whole drive....
> Is the tradeoff worth it? Now you have choices like Sata vs
> SAS vs SDD vs PCIe....

We have a multi-tiered storage architecture. When you want to play a video
from our service, we see if any of the close, fast boxes has a copy we can
use. If they are too busy, we go back to slower, but more complete tiers.
The last tier is made up of machines with lots of spinning disks. Some
catalogs are small enough that only using 1/2 the drive, but getting 30%
better throughput is the right engineering decision since it would improve
network utilization w/o needing to deploy more servers. We use all those
technologies in the different tiers: our fastest 100G boxes are NVMe, the
40G boxes we have are JBOD of SSDs, the 10G storage boxes are spinning
rust. We use SATA for SSDs since the SAS SSDs are super pricy, but we use
SAS HDDs since we need the deeper queues and other features of SAS that are
absent from SATA. We also sometimes oversubscribe PCIe lanes to get better
storage density at a cheaper price point.

There's lots of tradeoffs that can be made....

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