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Subject: HASt and write amplification on the seconday
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From: Pete French <petefrench@ingresso.co.uk>
Date: Mon, 30 Jan 2017 18:41:21 +0000
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I have a pair of machines which I have been runnig HAST on
for a number of years. It works well, it does what Ineed it
to do, and I havent considered the details until recently.

As I udnesratnd it though, the minimum size of block copied is
set by default to something quite arge (2 meg). I see whay, but
as I am using ZFS on top I am generating lots of writes which are
scattered across the disc (ZFS not known for keeping writes all in
the same place!).

So, is each 512 byte write being turned into a 2 meg write on the
secondary ? If so this worries me, as I am about to replcae these
drives with SSD's, which I do not want to wearn out with excessive
writes.

Am I worrying over nothing ? Should I try and tune the HAST sizes and the
zpool ashift to match, or will that trash performances 9some limited
experiments suggest it will).

-pete.