[linux-lvm] Looking ahead - tiering with LVM?

[Gionatan Danti]

Il 2020-09-09 21:53 John Stoffel ha scritto:
> Very true, numbers talk, annecdotes walk...

Sure - lets try to gather some numbers from the data you posted 
before...

> sudo lvcache status data/home
> +-----------------------+------------------+
> | Field                 | Value            |
> +-----------------------+------------------+
> | cached                | True             |
> | size                  | 806380109824     |
> | cache_lv              | home_cache       |
> | cache_lv_size         | 85899345920      |

You cache device is squarely in the 10x ballpark (ie: it is ~9.39x 
smaller than your SSD). Having ~10.64% more space would be nice, but 
hardly game-changer.

> | read_hits             | 138697828        |
> | read_misses           | 7874434          |

You have 94.6% hit rate from your hotspot cache - compare this to an 
ideally managed tiered storage, with its 100% (ideal, not reasonable in 
real world) hit rate. Does it really change anything?

> | write_hits            | 777455171        |
> | write_misses          | 9841866          |

And you have an even better ~98.7% write hit ratio. As you have a 
mirrored cache device, you should be able to set LVM for using a 
writeback cache without risking your data in case of a single cache 
drive failure. I suspect this would do wonder with your hit ratio - but 
again, testing is the only method to be sure.

Thanks for sharing your data!
Regards.

-- 
Danti Gionatan
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