Date: Thu, 24 Jan 2013 09:32:58 +0100 From: Borja Marcos <borjam@sarenet.es> To: Jean-Yves Moulin <jym@baaz.fr> Cc: FreeBSD Filesystems <freebsd-fs@freebsd.org>, Scott Long <scottl@samsco.org> Subject: Re: RFC: Suggesting ZFS "best practices" in FreeBSD Message-ID: <8BA7B786-3B4B-473B-B4F0-798C9B5AEF00@sarenet.es> In-Reply-To: <81460DE8-89B4-41E8-9D93-81B8CC27AA87@baaz.fr> References: <314B600D-E8E6-4300-B60F-33D5FA5A39CF@sarenet.es> <565CB55B-9A75-47F4-A88B-18FA8556E6A2@samsco.org> <81460DE8-89B4-41E8-9D93-81B8CC27AA87@baaz.fr>
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On Jan 23, 2013, at 4:16 PM, Jean-Yves Moulin wrote: > But what about battery-backed cache RAID card ? They offer a = non-volatile cache that improves writes. And this cache is safe because = of the battery. These feature doesn't exist on bare disks. They can be "fine" for certain applications, especially with limited = ability filesystems. But we are speaking about using maybe the latest = and greatest in filesystem technology, with a superior mechanism to manage redundancy = and I/O bandwidth. Using another redundancy mechanism underneath can make matters worse, with one system working against the other. ZFS manages it better. ZFS allows you to decide if you need to cache = metadata and/or data or none of them. RAID cards can show stupid caching = behaviors depending on your workload. So, RAID card with ZFS, definitely a no-no. As Scott said, more failure = modes. And some of them, complex. Many trivial operations may require a = reboot. The card hides important disk diagnostics from ZFS. Borja.
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