I am trying to come up with an algorithm to divide a single fast block
device into cache data and metadata, and I have several questions.
1. Is metadata_size = 4MiB + 16 * nr_blocks still the correct formula
for the metadata size?
2. Am I correct in thinking that the number of blocks is based on the
size of the cache data, rather than the origin device -- i.e.
nr_blocks = cache_size / block_size?
3. Assuming that both of the above are correct, how does this look?
AS: available space on "fast device"
BS: block size
MS: metadata size
CS: cache data size
NB: number of blocks
NB = CS / BS
MS = 4MiB + 16 * NB
= 4MiB + 16 * (CS / BS)
CS = AS - MS
= AS - (4MiB + 16 * (CS / BS))
= AS - 4MiB - 16 * CS / BS
CS * BS = (AS - 4MiB) * BS - 16 * CS
CS * BS + 16 * CS = (AS - 4MiB) * BS
CS * (BS + 16) = (AS - 4MiB) * BS
CS = (AS - 4MiB) * BS / (BS + 16)
I.e. given the available space on my fast device, and the desired
block size, I can calculate the cache data size as:
cache_size = (available_size - 4MiB) * block_size /
(block_size + 16)
and:
metadata_size = available_size - cache_size
Yay algebra!
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Ian Pilcher arequipeno@xxxxxxxxx
-------- "I grew up before Mark Zuckerberg invented friendship" --------
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