Date: Mon, 30 Jan 2017 18:41:21 +0000 From: Pete French <petefrench@ingresso.co.uk> To: stable@freebsd.org Subject: HASt and write amplification on the seconday Message-ID: <E1cYGtF-000I5u-Sz@dilbert.ingresso.co.uk>
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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.
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