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Re: hole punching performance

To: Dave Chinner <david@xxxxxxxxxxxxx>
Subject: Re: hole punching performance
From: "Bradley C. Kuszmaul" <kuszmaul@xxxxxxxxx>
Date: Tue, 15 Jan 2013 18:33:54 -0500
Cc: Florian Weimer <fw@xxxxxxxxxxxxx>, Ben Myers <bpm@xxxxxxx>, xfs@xxxxxxxxxxx
Delivered-to: xfs@xxxxxxxxxxx
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In-reply-to: <20130115225435.GB2498@dastard>
References: <CAKSyJXf66H2U-BF-aYnSr2fF24_6LJw6swOx1RhUc_3Eqayaiw@xxxxxxxxxxxxxx> <87pq195t8q.fsf@xxxxxxxxxxxxxxxxx> <CAKSyJXfQsmemZ3H9A92UezsPCXfn37LXP3tpPMkiwtvR4=eY2g@xxxxxxxxxxxxxx> <20130113194941.GB27055@xxxxxxx> <20130113225545.GA19173@dastard> <87mwwaqgkj.fsf@xxxxxxxxxxxxxxxxx> <CAKSyJXdejNhNgqq6g8N4n6Cjz2s3+m=JUvanTRfDtF72d2RxFg@xxxxxxxxxxxxxx> <20130115225435.GB2498@dastard>
So my understanding is that the first time I do a read() I'll have to read the extents.  My writes are on the order of a megabyte, a 250MB file will likely require an extra I/O to open.  That's fine.  

It really has been helpful to have all this performance-modeling information.  

Thanks.

-Bradley

On Tue, Jan 15, 2013 at 5:54 PM, Dave Chinner <david@xxxxxxxxxxxxx> wrote:
On Tue, Jan 15, 2013 at 04:34:50PM -0500, Bradley C. Kuszmaul wrote:
> So does it cost a lot to read all the extents just to read the first block?

Depends on the number of extents in the file. Figure an IO per 250
extents for a 4k block size filesystem (extent records are 16 bytes
in size on disk).

Cheers,

Dave.
--
Dave Chinner
david@xxxxxxxxxxxxx

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