Messages in this thread | | | Date | Tue, 10 Apr 2012 10:21:48 -0400 | From | Vivek Goyal <> | Subject | Re: [RFC PATCH] block: Change default IO scheduler to deadline except SATA |
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On Tue, Apr 10, 2012 at 03:56:39PM +0200, Jens Axboe wrote: > On 2012-04-10 15:37, Vivek Goyal wrote: > > Hi, > > > > I am wondering if CFQ as default scheduler is still the right choice. CFQ > > generally works well on slow rotational media (SATA?). But often > > underperforms on faster storage (storage arrays, PCIE SSDs, virtualized > > disk in linux guests etc). People often put logic in user space to tune their > > systems and change IO scheduler to deadline to get better performance on > > faster storage. > > > > Though there is not one good answer for all kind of storage and for all > > kind of workloads, I am wondering if we can provide a better default and > > that is change default IO scheduler to "deadline" except SATA. > > > > One can argue that some SAS disks can be slow too and benefit from CFQ. Yes, > > but default IO scheduler choice is not perfect anyway. It just tries to > > cater to a wide variety of use cases out of the box. > > > > So I am throwing this patch out see if it flies. Personally, I think it > > might turn out to be a more reasonable default. > > I think it'd be a lot more sane to just use CFQ on rotational single > devices, and default to deadline on raid or non-rotational devices. This > still isn't perfect, since less worthy SSDs still benefit from the > read/write separation, and some multi device configs will be faster as > well. But it's better.
Hi Jens,
Thanks. Taking a decision based on rotational flag makes sense. I am not sure that does one get the information that a block device is a single device or not. Especially with HBAs, SCSI Luns over Fiber, iSCSI Luns etc. I have few Scsi Luns exported to me backed by a storage array. Everything runs CFQ by default. And though disks in the array are rotational, they are RAIDed and AFAIK, this information is not available to driver.
I am not sure if there is an easy way to get similar info for dm/md devices.
> > The below patch is not a good idea. There's no clear distinction between > on what CFQ is now the default.
Can it be thought of as that block layer default is "deadline" and driver can override it. Yes, I agree that it is not very clear though.
Thanks Vivek
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