[dm-devel] dm-cache: can the same cache be used with multiple origin devices?

[Mears, Morgan]

Hi,

In reference to dm-cache: can the same cache and metadata devices be used with multiple origin devices?  This can be configured, and we've done some tests that appear to show that it works - we're looking for confirmation (or otherwise).

Here's an example test setup to clarify -- ssd_metadata and ssd_blocks are being used to cache sdc and sdd.  In testing, different patterns were written to areas of sdc_cached and  sdd_cached; afterwards, the contents of sdc and sdd were as expected.

dmsetup create sdc_cached --table '0 4194304 cache /dev/mapper/ssd_metadata /dev/mapper/ssd_blocks /dev/sdc 512 1 writethrough default 0'
dmsetup create sdd_cached --table '0 4194304 cache /dev/mapper/ssd_metadata /dev/mapper/ssd_blocks /dev/sdd 512 1 writethrough default 0'

Our thinking is that using one large cache for multiple origin devices will result in a more efficient use of flash resources than statically partitioning the flash amongst the origins.  Also, there's no need to repartition or buy another SSD to start caching a newly-added origin.

Thanks,
Morgan Mears