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List:       zfs-discuss
Subject:    Re: [zfs-discuss] SAS vs SATA: Same size, same speed, why SAS?
From:       Brandon High <bhigh () freaks ! com>
Date:       2010-04-28 1:03:15
Message-ID: s2need2751f1004271803m842ab371o57aaebe38ddb5e45 () mail ! gmail ! com
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On Tue, Apr 27, 2010 at 2:47 PM, Daniel Carosone <dan@geek.com.au> wrote:
>> What's so lame about NCQ?
>
> Primarily, the meager number of outstanding requests; write cache is
> needed to pretend the writes are done straight away and free up the
> slots for reads.

NCQ handles 32 outstanding operations. Considering that ZFS limits the
outstanding requests to 10 (as of snv_125 I think?), that's not an
issue. TCQ supports between 16 and 64 bits for the tags, depending on
the implementation and underlying protocol.

TCQ allows a command to be added to the as head of the queue, ordered,
or simple. I don't believe that NCQ allows multiple queuing methods,
and I can't be bothered to check.

-B

-- 
Brandon High : bhigh@freaks.com
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