TRIM, iSCSI and %busy waves

Steven Hartland killing at
Fri Apr 6 08:41:38 UTC 2018

That is very hw and use case dependent.

The reason we originally sponsored the project to add TRIM to ZFS was that
in our case without TRIM the performance got so bad that we had to secure
erase disks every couple of weeks as they simply became so slow they where

Now admittedly that was a fair few years ago and hw has moved on since
then, but the point remains that it’s not totally true that just not
TRIMing is an option.

On Fri, 6 Apr 2018 at 09:10, Borja Marcos <borjam at> wrote:

> > On 5 Apr 2018, at 17:00, Warner Losh <imp at> wrote:
> >
> > I'm working on trim shaping in -current right now. It's focused on NVMe,
> > but since I'm doing the bulk of it in cam_iosched.c, it will eventually
> be
> > available for ada and da. The notion is to measure how long the TRIMs
> take,
> > and only send them at 80% of that rate when there's other traffic in the
> > queue (so if trims are taking 100ms, send them no faster than 8/s). While
> > this will allow for better read/write traffic, it does slow the TRIMs
> down
> > which slows down whatever they may be blocking in the upper layers. Can't
> > speak to ZFS much, but for UFS that's freeing of blocks so things like
> new
> > block allocation may be delayed if we're almost out of disk (which we
> have
> > no signal for, so there's no way for the lower layers to prioritize trims
> > or not).
> Have you considered "hard" shaping including discarding TRIMs when needed?
> Remember that a TRIM is not a write, which is subject to a contract with
> the application,
> but a better-if-you-do-it operation.
> Otherwise, as you say, you might be blocking other operations in the upper
> layers.
> I am assuming here that with many devices doing TRIMs is better than not
> doing them.
> And in case of queue congestion doing *some* TRIMs should be better than
> doing
> no TRIMs at all.
> Yep, not the first time I propose something of the sort, but my queue of
> suggestions
> to eventually discard TRIMs doesn’s implement the same method ;)
> Borja.
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