Date: Tue, 19 Oct 2010 12:54:03 +0100 From: Pete French <petefrench@ticketswitch.com> To: pjd@freebsd.org, zbeeble@gmail.com Cc: freebsd-fs@freebsd.org Subject: Re: Converting a non-HAST ZFS pool to a HAST pool Message-ID: <E1P8AlX-0001P4-KI@dilbert.ticketswitch.com> In-Reply-To: <AANLkTi=55qO5uqfv3rkf8SncEo5syznSH%2BTgyhOGTHiz@mail.gmail.com>
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> I'm wondering if I'm missing something here --- because I'm wondering > if running HAST under ZFS isn't a step backwards. > > My quick read of HAST seems to indicate that it's going to manage two > disks and present them as one disk to ZFS. The design problem with > this (especially since we're talking a _lot_ of network (and memory) > transfers involved) is data corruption --- the idea that ZFS protects > data better when it can determine one disk has it right while another > disk has it wrong (as it can when it manages the two disks). > > Wouldn't it be better to just have network (iscsi-like) spools > attached to ZFS? Individual spools could still fail. What am I > missing? Is there a better description of HAST than the FreeBSD wiki > page? I guess in theory the answer there is 'yes' - but have you actually tried it in practice ? I did this for a while as an experiment using ggated - the problem is that when the remote fails (for example) it doesnt signal up to ZFS properly and instead of ZFS seeing a failed driev it just locks up. iSCSI had similar issues. Note that this test was a while ago, and the situation may have improved - but that was the way I did it at the time I set this up. Interstingly, I am considering a hybrid - using ZFS as a mirror over a pair of HAST devices. My servers have hardwar RAID in them and a pair of drives each, so I could split those, remove the hardware RAID, and run 4 drives as 2 hast devices with ZFS on top. That soulds like the best soultuon to me - but you do then have the issue of doubling your network bandwidth required. -pete.
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