On Mon, Apr 30, 2018 at 12:07:31PM -0600, Jens Axboe wrote: > On 4/30/18 11:19 AM, Brian Foster wrote: > > On Mon, Apr 30, 2018 at 09:32:52AM -0600, Jens Axboe wrote: > >> XFS recently added support for async discards. While this can be > >> a win for some workloads and devices, there are also cases where > >> async bursty discard will severly harm the latencies of reads > >> and writes. > >> > >> Add a 'discard_sync' mount flag to revert to using sync discard, > >> issuing them one at the time and waiting for each one. This fixes > >> a big performance regression we had moving to kernels that include > >> the XFS async discard support. > >> > >> Signed-off-by: Jens Axboe <axboe@xxxxxxxxx> > >> --- > > > > Hm, I figured the async discard stuff would have been a pretty clear win > > all around, but then again I'm not terribly familiar with what happens > > with discards beneath the fs. I do know that the previous behavior would > > cause fs level latencies due to holding up log I/O completion while > > discards completed one at a time. My understanding is that this lead to > > online discard being pretty much universally "not recommended" in favor > > of fstrim. > > It's not a secret that most devices suck at discard. While the async > discard is nifty and I bet works well for some cases, it can also cause > a flood of discards on the device side which does not work well for > other cases. > Heh, Ok. > > Do you have any more data around the workload where the old sync discard > > behavior actually provides an overall win over the newer behavior? Is it > > purely a matter of workload, or is it a workload+device thing with how > > discards may be handled by certain devices? > > The worse read latencies were observed on more than one device type, > making it sync again was universally a win. We've had many issues > with discard, one trick that is often used is to chop up file deletion > into smaller chunks. Say you want to kill 10GB of data, you do it > incrementally, since 10G of discard usually doesn't behave very nicely. > If you make that async, then you're back to square one. > Makes sense, so there's not much win in chopping up huge discard ranges into smaller, async requests that cover the same overall size/range.. > > I'm ultimately not against doing this if it's useful for somebody and is > > otherwise buried under a mount option, but it would be nice to see if > > there's opportunity to improve the async mechanism before resorting to > > that. Is the issue here too large bio chains, too many chains at once, > > or just simply too many discards (regardless of chaining) at the same > > time? > > Well, ultimately you'd need better scheduling of the discards, but for > most devices what really works the best is a simple "just do one at > the time". The size constraint was added to further limit the impact. > ... but presumably there is some value in submitting some number of requests together provided they adhere to some size constraint..? Is there a typical size constraint for the average ssd, or is this value all over the place? (Is there a field somewhere in the bdev that the fs can query?) (I guess I'll defer to Christoph's input on this, I assume he measured some kind of improvement in the previous async work..) > Honestly, I think the only real improvement would be on the device > side. Most folks want discard as an advisory hint, and it should not > impact current workloads at all. In reality, many implementations > are much more strict and even include substantial flash writes. For > the cases where we can't just turn it off (I'd love to), we at least > need to make it less intrusive. > > > I'm wondering if xfs could separate discard submission from log I/O > > completion entirely and then perhaps limit/throttle submission somehow > > or another (Christoph, thoughts?) via a background task. Perhaps doing > > something like that may even eliminate the need for some discards on > > busy filesystems with a lot of block free -> reallocation activity, but > > I'm just handwaving atm. > > Just having the sync vs async option is the best split imho. The async > could potentially be scheduled. I don't think more involved logic > belongs in the fs. > The more interesting part to me is whether we can safely separate discard from log I/O completion in XFS. Then we can release the log buffer locks and whatnot and let the fs proceed without waiting on any number of discards to complete. In theory, I think the background task could issue discards one at a time (or N at a time, or N blocks at a time, whatever..) without putting us back in a place where discards hold up the log and subsequently lock up the rest of the fs. If that's possible, then the whole sync/async thing is more of an implementation detail and we have no need for separate mount options for users to try and grok. Brian > -- > Jens Axboe > > -- > To unsubscribe from this list: send the line "unsubscribe linux-xfs" in > the body of a message to majordomo@xxxxxxxxxxxxxxx > More majordomo info at http://vger.kernel.org/majordomo-info.html
