Date: Tue, 29 Oct 2002 13:09:43 +1100 From: Peter Jeremy <peter.jeremy@alcatel.com.au> To: "Wilkinson,Alex" <Alex.Wilkinson@dsto.defence.gov.au> Cc: alpha@FreeBSD.ORG Subject: Re: [hardware] Tagged Command Queuing or Larger Cache ? Message-ID: <20021029020943.GI6446@gsmx07.alcatel.com.au> In-Reply-To: <20021029095516.G91719-100000@squirm.dsto.defence.gov.au> References: <20021029095516.G91719-100000@squirm.dsto.defence.gov.au>
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On 2002-Oct-29 09:55:49 +1030, "Wilkinson,Alex" <Alex.Wilkinson@dsto.defence.gov.au> wrote: >Do the benifits from a having a larger disk cache such as the "WD >40GB 7200RPM w/8MB Cache" has, outweigh the benefits of Tagged >Command Queuing ? This is somewhat off-topic for -alpha... The quick answer is: "Not if you value your data". The longer answer is: Without tagged queueing, the device driver cannot determine when the data for a particular write command has been committed to the media (as against just being stored in the cache). Soft-updates (in particular) relies on accurate write ordering to ensure that the filesystem on disk is always consistent. Having a volatile cache in the disk breaks this assumption - if you have a power failure, you don't know what was lost. The safe states are: tagging & caching or no-caching (which is slow). Peter To Unsubscribe: send mail to majordomo@FreeBSD.org with "unsubscribe freebsd-alpha" in the body of the message
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