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Re: [PATCH] xfs_repair: multithread phase 2

To: xfs@xxxxxxxxxxx
Subject: Re: [PATCH] xfs_repair: multithread phase 2
From: Michael Monnerie <michael.monnerie@xxxxxxxxxxxxxxxxxxx>
Date: Mon, 10 Jan 2011 14:25:47 +0100
Cc: Dave Chinner <david@xxxxxxxxxxxxx>
In-reply-to: <20110110084122.GF28803@dastard>
Organization: it-management http://it-management.at
References: <1294620248-17098-1-git-send-email-david@xxxxxxxxxxxxx> <201101100857.53421@xxxxxx> <20110110084122.GF28803@dastard>
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On Montag, 10. Januar 2011 Dave Chinner wrote:
> Pretty much
> every sata disk supports NCQ these days, and default to a depth of
> 32, which means we can have 32 concurrent reads in progress at once.
> Phase 2 is all synchronous IO, so the only way to hide the IO
> latency is to queue work to multiple threads and switch between the
> threadsto work on another queue when the current one blocks waiting
> for IO.

This is interesting. Did you measure this with a rotating single disk? 
Is the idle time between two synchronous reads bigger than the time 
needed to move the disk head to another cylinder and read a sector? That 
takes ~15ms on a normal disk, incredibly long compared to cpu speed. 
Even with NCQ, the disk would have to swing the head a lot, and just 
from thinking about it I wouldn't believe that it's faster like this.

But I'm sure you tested it so I take it as given that it's like that. 
Cool improvement, btw :-)

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