Re: [RFC] Add sysctl option to drop disk flushes in bcache? (was: Bcache in writes direct with fsync)

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On Tue, May 24, 2022 at 01:14:18PM -0700, Eric Wheeler wrote:
> Hi Christoph,
> 
> On Mon, 23 May 2022, Christoph Hellwig wrote:
> > ... wait.
> > 
> > Can someone explain what this is all about?  Devices with power fail 
> > protection will advertise that (using VWC flag in NVMe for example) and 
> > we will never send flushes. So anything that explicitly disables flushed 
> > will generally cause data corruption.
> 
> Adriano was getting 1.5ms sync-write ioping's to an NVMe through bcache 
> (instead of the expected ~70us), so perhaps the NVMe flushes were killing 
> performance if every write was also forcing an erase cycle.
> 
> The suggestion was to disable flushes in bcache as a troubleshooting step 
> to see if that solved the problem, but with the warning that it could be 
> unsafe.
> 
> Questions:
> 
> 1. If a user knows their disks have a non-volatile cache then is it safe 
>    to drop flushes?
> 
> 2. If not, then under what circumstances is it unsafe with a non-volatile 
>    cache?
>   
> 3. Since the block layer wont send flushes when the hardware reports that 
>    the cache is non-volatile, then how do you query the device to make 
>    sure it is reporting correctly?  For NVMe you can get VWC as:
> 	nvme id-ctrl -H /dev/nvme0 |grep -A1 vwc
>    
>    ...but how do you query a block device (like a RAID LUN) to make sure 
>    it is reporting a non-volatile cache correctly?

You can check the queue attribute, /sys/block/<disk>/queue/write_cache. If the
value is "write through", then the device is reporting it doesn't have a
volatile cache. If it is "write back", then it has a volatile cache.



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