Hi,
We are a group of scientists, who work on reasonably sized datasets (10-100GB). Because we had troubles managing our SSD's (everyone likes to have their data on the SSD), I set up a caching system where the 500GB SSD caches the 4TB HD. This way, everybody would have their data virtually on the SSD, and only the first pass through the dataset would be slow. Afterwards, it would be cached anyway, and the reads would be faster.
I used lvm-cache for this. Yet, it seems that the (only) smq-policy is very reluctant in promoting data to the cache, whereas what we would need, is that data is promoted basically upon the first read. Because if someone is using the machine on certain data, they will most likely go over the dataset a couple of hundred times in the following hours.
Right now, after a week of testing lvm-cache with the smq-policy, it looks like this:
jdgrave@kat:~$ sudo ./lvmstats
start 0
end 7516192768
segment_type cache
md_block_size 8
md_utilization 14353/1179648
cache_block_size 128
cache_utilization 7208960/7208960
read_hits 19954892
read_misses 84623959
read_hit_ratio 19.08%
write_hits 672621
write_misses 7336700
write_hit_ratio 8.40%
demotions 151757
promotions 151757
dirty 0
features 1
jdgrave@kat:~$ sudo ./lvmcache-statistics.sh
-------------------------------------------------------------------------
LVM [2.02.133(2)] cache report of found device /dev/VG/lv
-------------------------------------------------------------------------
- Cache Usage: 100.0% - Metadata Usage: 1.2%
- Read Hit Rate: 19.0% - Write Hit Rate: 8.3%
- Demotions/Promotions/Dirty: 151757/151757/0
- Feature arguments in use: writeback
- Core arguments in use : migration_threshold 2048 smq 0
- Cache Policy: stochastic multiqueue (smq)
- Cache Metadata Mode: rw
- MetaData Operation Health: ok
The number of promotions has been very low, even though the read hit rate is low as well. This is with a cache of 450GB, and currently only 614GB of data on the cached device. A read hit rate of lower than 20%, when just randomly caching would have achieved 73% is not what I would have hoped to get.
Is there a way to make the caching way more aggressive? Some settings I can tweak?
Yours sincerely,
Jonas
Jonas
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