Re: 20TB Storage System

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Date: Fri, 05 Sep 2003 22:03:39 -0700
To: David Gilbert <dgilbert@dclg.ca>

David Gilbert wrote:
> >>>>> "Poul-Henning" == Poul-Henning Kamp <phk@phk.freebsd.dk> writes:
> Poul-Henning> I am not sure I would advocate 64k blocks yet.
> Poul-Henning> I tend to stick with 32k block, 4k fragment myself.
>
> That reminds me... has anyone thought of designing the system to have
> more than 8 frags per block? Increasingly, for large file
> performance, we're pushing up the block size dramatically. This is
> with the assumption that large disks will contain large files.

My assumptions on the previous two statements by Poul are:

1) You cannot trust that a short will be treated as an
        unsigned 16 bit value in all cases, so values that
        are between 32768 and 65535 may be treated incorrectly.

2) A fully populate block bitmap byte, which means a divide
        by 8, is necessary to avoid potential division errors.

In other words, he's afraid that the sign bit and/or the block
size bitmap used by frags may be treated incorrectly.

I have to agree with both those observations. A number of people
have, historically, reported issues with a divisor other than 8,
and the worry about the sign bit is common sense, given the many
historical issues faced by other OS's when it comes to 64K block
sizes.

-- Terry
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• Previous message: David Schultz: "Re: 20TB Storage System"
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