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Message-ID: <543C64AA.1000307@bluerosetech.com>
Date: Mon, 13 Oct 2014 16:47:54 -0700
From: Darren Pilgrim <list_freebsd@bluerosetech.com>
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To: "Matthew D. Fuller" <fullermd@over-yonder.net>
Subject: Re: getting to 4K disk blocks in ZFS
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On 10/13/2014 12:19 PM, Matthew D. Fuller wrote:
> On Mon, Oct 13, 2014 at 11:48:27AM -0700 I heard the voice of
> Darren Pilgrim, and lo! it spake thus:
>>
>> If the default is 4k and (for the limited time they're still common)
>> you use true 512b disks, you can waste space.  Sure, but how much
>> space?
>
> The median file in /usr/ports is 408 bytes.  Over 90% of the files are
> under 2k, which means the wastage for them is over 100% (before
> counting what gain compression might get).  A little offhand mathery
> says it's about 78% extra overhead on the whole.
>
> And that includes the almost hundred megs (over 22% of the total size
> of the FS) for the INDEX.db, plus the ~90 megs of the flat INDEX files
> (another 20%).  If you pull those out, the overhead is 130%.

The worst case is about 140%.  But that's 140% of 388 MB, so we go from 
tiny to somewhat less tiny.  Huge slack is expected in small, populous 
datasets.  I'd much rather have the integrity assurance and performance 
gains, instead of saving a few hundred MB out of a multi-terabyte pool.