Re: Re: [PATCH] bcache: add a separate open bucket list for flash devices

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From: Tang Junhui <tang.junhui@xxxxxxxxxx>

On Tue, Nov 21, 2017 at 06:50:32PM +0800, tang.junhui@xxxxxxxxxx wrote:
> > From: Tang Junhui <tang.junhui@xxxxxxxxxx>
> > 
> > Currently in pick_data_bucket(), though we keep multiple buckets open
> > for writes, and try to segregate different write streams for better
> > cache utilization: first we look for a bucket where the last write to
> > it was sequential with the current write, and failing that we look
> > for a bucket that was last usedby the same task.
> > 
> > But actually, in such scenario that there are some flash devices
> > , and some backend bcache devices, when many tasks request these
> > flash devices and backend bcache devices, the write IOs may still
> > fall to the same bucket as bellow bucket:
> > | bcache data | flash data | bcache data | bcache data| flash data|
> > then after writeback of these backend bcache devices, the bucket would
> > be like bellow bucket:
> > | free | flash data | free | free | flash data |

> This patch doesn't make any sense - open_buckets are only for allocations and
> writes to the SSD.

Thank you for your quick respond, It's true that open_buckets are only for 
allocations and writes to the SSD, but in writeback mode, data from flash 
devices and backend bcache devices are all write to bucket(SSD). This patch add 
a separate open bucket list for flash devices, so data from flash devices 
and backend bcache devices can store in different buckets. 

Thanks,
Tang





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