getting to 4K disk blocks in ZFS

[krad]

your also forgetting you can enable compression, which on mail files will
give you a large ratio

On 13 October 2014 21:47, Don Lewis <truckman at freebsd.org> wrote:

> On 13 Oct, Steven Hartland wrote:
> > ----- Original Message -----
> > From: "Matthew D. Fuller" <fullermd at over-yonder.net>
> >
> >
> >> 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%.
> >>
> >>
> >> (To be sure, relatively few people have ports trees eating most of
> >> their space, but still; it's pretty pathological.  I for one did
> >> decide some years back to always force 4k on any new FSen to make
> >> future life simpler, accepting the bloat, but it's there.)
> >
> > And thats before you add the overhead if your running RAIDZ...
> >
> > A good read on this is
> > http://blog.delphix.com/matt/2014/06/06/zfs-stripe-width/
>
> This is a timely subject.  I'm planning on moving my Cyrus imap mail
> spool from a 4K/1K UFS filesystem to a three drive raidz1.  It looks
> like the UFS fragmentation overhead is about 2.4%.  ZFS ashift=12
> increases that to about 17%.  Combine that with raidz and now the
> overhead is about 40%.  Ouch!
>
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