On Thu, Jul 11, 2013 at 1:52 PM, sankalp kohli <kohlisank...@gmail.com>wrote:
> Scrub will keep the file size same. YOu need to move all sstables to be
> L0. the way to do this is to remove the json file which has level
> information.
>
This will work, but I believe is subject to this?
"./src/java/org/apache/cassandra/db/compaction/LeveledManifest.java" line
228 of 577
"
// LevelDB gives each level a score of how much data it contains vs
its ideal amount, and
// compacts the level with the highest score. But this falls apart
spectacularly once you
// get behind. Consider this set of levels:
// L0: 988 [ideal: 4]
// L1: 117 [ideal: 10]
// L2: 12 [ideal: 100]
//
// The problem is that L0 has a much higher score (almost 250) than
L1 (11), so what we'll
// do is compact a batch of MAX_COMPACTING_L0 sstables with all 117
L1 sstables, and put the
// result (say, 120 sstables) in L1. Then we'll compact the next
batch of MAX_COMPACTING_L0,
// and so forth. So we spend most of our i/o rewriting the L1 data
with each batch.
//
// If we could just do *all* L0 a single time with L1, that would
be ideal. But we can't
// -- see the javadoc for MAX_COMPACTING_L0.
//
// LevelDB's way around this is to simply block writes if L0
compaction falls behind.
// We don't have that luxury.
//
// So instead, we force compacting higher levels first. This may
not minimize the number
// of reads done as quickly in the short term, but it minimizes the
i/o needed to compact
// optimially which gives us a long term win.
"
Ideal would be something like a major compaction for LCS which allows end
user to change resulting SSTable sizes without forcing everything back to
L0.
=Rob
