It means that ES works well with SSDs since Lucene is write-once under the
hood, so it is "easy" on the SSDs, vs other approaches which do random
writes to different places causing the higher write amplification.
But, this is balanced with the fact that Lucene must also periodically
merge the segments, which is in fact its own higher level form of write
amplification: when you first index a doc, it's written into a new segment,
but over that doc's lifetime in the index it may be copied another 4-5
times or something before it lives in a "max sized" segment. Still, that
higher write amplification likely works out to much less stress on the SSD
than databases that do random writes to their stores.
Mike McCandless
http://blog.mikemccandless.com
On Tue, Dec 16, 2014 at 4:11 AM, AndrewK kenworthyas@gmail.com wrote:
At the core training I attended last year there was a side note on SSD and
write amplification: roughly along the lines of: write amplification can be
a big problem with SSD (as writes can be around 4KB but deletes are often
in blocks of around 512KB, and that the problem gets worse the smaller and
the more random the writes are), but that write amplification is never an
issue in ES as all writes are sequential anyway (reading from my notes
here).What does that mean exactly? That write amplification can be a big
problem with SSD, but not with ES on SSD, or that the problem is relevant
with lots of random writes? (I suspect the former, but am not quite sure).--
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