Date: Mon, 31 Mar 2008 23:34:04 +0200 From: "Ivan Voras" <ivoras@freebsd.org> To: "Scott Long" <scottl@samsco.org> Cc: freebsd-current@freebsd.org Subject: Re: Are large RAID stripe sizes useful with FreeBSD? Message-ID: <9bbcef730803311434s48d3269cs1e8ae0fd1eb7ffc3@mail.gmail.com> In-Reply-To: <47F15772.5010104@samsco.org> References: <fsr7fb$hl0$1@ger.gmane.org> <47F147D8.3030905@samsco.org> <9bbcef730803311409ha25effam9dd522c9084783ad@mail.gmail.com> <47F15772.5010104@samsco.org>
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On 31/03/2008, Scott Long <scottl@samsco.org> wrote: > For writes, the performance penalty of smaller I/O's (assuming no RAID-5 > effects) is minimal; most caching controllers and drives will batch the > concurrent requests together, so the only loss is in the slight overhead > of the extra transaction setup and completion. For reads, the penalty > can be greater because the controller/disk will try to execute the first > request immediately and not wait for the second part to be requested, > leading to the potential for extra rotational and head movement delays. > Many caching RAID controllers offer a read-ahead feature to counteract > this. However, while my testing has shown little measurable benefit to > this, YMMV. Thank you, this is the kind of explanation I hoping for. One more thing: is TCQ (e.g. the SCSI variant) orthogonal to this?
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