dm-cache metadata memory footprint

dm-cache metadata memory footprint

[Thanos Makatos]

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Is there some formula that could indicate the metadata memory footprint of
dm-cache for given SSD and HDD capacities? Is there anything else that
would affect the metadata footprint (e.g. write-through vs. write back,
cache line size, etc.)?

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Re: dm-cache metadata memory footprint

[Thanos Makatos]

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Ping!

I'd like to generalize my question to what is the general memory footprint
of dm-cache. Does dm-cache keeps all metadata in memory or only part of it?
Are there any other memory overheads? Are the metadata pinned or
dynamically cached, according to available memory? I need to know such
information in order to tell whether my system has currently enough
resources to create a dm-cache target.

I understand that the metadata size is dictated by the size of the SSD and
the size of the disk doesn't have anything to do with it. To test how
memory utilization is affected, I created one hundred 1 GB dm-cache targets
using 128 KB cache block size. I calculated the size of the metadata
according to
https://www.redhat.com/archives/dm-devel/2012-December/msg00046.html, which
is 4.12 MB per dm-cache target. Memory utilization did increase but not as
much as I expected: it increased by 183 MB and not by 4.12*100 which would
be the case if all metadata were kept in memory. (I didn't do any I/O to
the dm-cache targets.)

If this information is not readily available, are there any specific parts
in the code I should focus in order to figure  this out?

On Tue, Feb 17, 2015 at 12:22 PM, Thanos Makatos <thanos.makatos@onapp.com>
wrote:

> Is there some formula that could indicate the metadata memory footprint of
> dm-cache for given SSD and HDD capacities? Is there anything else that
> would affect the metadata footprint (e.g. write-through vs. write back,
> cache line size, etc.)?
>

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