[dm-devel] [PATCH RFC 0/2] dm-zoned: add cache device

[Hannes Reinecke]

On 3/23/20 4:15 PM, Mike Snitzer wrote:
> On Mon, Mar 23 2020 at 11:03am -0400,
> Hannes Reinecke <hare at suse.de> wrote:
> 
>> Hi Damien,
>>
>> as my original plan to upgrade bcache to work for SMR devices
>> turned out to be more complex than anticipated I went for the
>> simpler approach and added a 'cache' device for dm-zoned.
>> It is using a normal device (eg '/dev/pmem0' :-), split it
>> into zones of the same size of the original SMR device, and
>> makes those 'virtual' zones avialable to dm-zoned in a similar
>> manner than the existing 'random write' zoned.
>>
>> The implementation is still a bit rough (one would need to add
>> metadata to the cache device, too), but so far it seems to work
>> quite well; still running after copying 300GB of data back and forth.
>>
>> As usual, comments and reviews are welcome.
> 
> Not seeing why this needs to be so specialized (natively implemented in
> dm-zoned).  Did you try stacking dm-writecache on dm-zoned?
> 
dm-zoned is using the random-write zones internally to stage writes to 
the sequential zones, so in effect it already has an internal caching.
All this patch does is to use a different device for the already present
mechanism.
Using dm-writecache would be ignorant of that mechanism, leading to 
double caching and detrimental results.

Cheers,

Hannes
-- 
Dr. Hannes Reinecke            Teamlead Storage & Networking
hare at suse.de                               +49 911 74053 688
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