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I've been running flashcache for a while now on both SSD and ZRAM devices. There's definitely a big improvement under the right situation. One of the biggest improvements I've seen so far has been running it against an ext4 LVM2 partition running a minecraft server:
0 4194304 flashcache stats:
read hits(986945), read hit percent(63)
write hits(10820089) write hit percent(91)
replacement(524380) write replacement(963051)
write invalidates(0) read invalidates(1)
pending enqueues(0) pending inval(0)
disk reads(566463) disk writes(10820089) ssd reads(986945) ssd writes(12373057)
uncached reads(1) uncached writes(0), uncached IO requeue(0)
pid_adds(0) pid_dels(0) pid_drops(0) pid_expiry(0)
If you get high enough cache reads and writes then it makes a world of difference.
Would be nice to see some hard and fast numbers though.
I also replaced tmpfs with zramfs a while ago on and saw substantial improvements there as well.
In light of OCZ releasing their new Synapse drive with a proprietary Windows-only caching software (review), I started to look for a Linux alternative for using a SSD as a block-level cache for a HDD and stumbled upon bcache. (Yes, bcache has been mentioned earlier in this thread too). At first I was a bit disappointed because there seemed to be some major tradeoffs, e.g. lacking TRIM. Thankfully only the bcache wiki was out of date and support for TRIM has been completed. The code has been rebased for 3.1 and bcache looks really promising. The lkml response hasn't been very warm so far (surprise ), but this thread is worth checking out anyway. Hopefully Michael will also test bcache and maybe give it some publicity in form of a news posting.