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From: David Magda <dmagda@ee.ryerson.ca>
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To: Bob Friesenhahn <bfriesen@simple.dallas.tx.us>
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	Jeremy Chadwick <freebsd@jdc.parodius.com>
Subject: Re: HEADS UP: ZFS v28 merged to 8-STABLE
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On Jun 10, 2011, at 17:24, Bob Friesenhahn wrote:

> Dedup can require a huge amount of RAM, or a dedicated L2ARC SSD, =
depending on the size of your storage.  You should not enable it unless =
you are prepared for the consequences.

Under OpenSolaris, each tracking entry for a deduped block (which can be =
between 512B to 128KB) can be up to 376 bytes (struct ddt_entry): so for =
one 1 GB (10^9) of deduped data (244140 blocks@4K), you would need ~91MB =
of overhead to keep track of it; for 1 TB (10^12) of deduped data  you =
would need ~91 GB of space to keep track of all the blocks. And if you =
can't fit the DDT in RAM, it will have to be saved to disk, which means =
more I/O to fetch the data.

=
http://www.freebsd.org/cgi/cvsweb.cgi/src/sys/cddl/contrib/opensolaris/uts=
/common/fs/zfs/sys/ddt.h?rev=3D1.2

If your data is in blocks smaller than 4K you'll need more memory for =
the DDT; if the data is broken up into blocks larger than 4K you'll =
probably need less.

Also remember that even though an L2ARC cache may save you from having =
to go to spinning rust, you still need to use some RAM (struct =
arc_buf_hdr; ~178B) to reference the DDT stuff in L2ARC:

=
http://www.freebsd.org/cgi/cvsweb.cgi/src/sys/cddl/contrib/opensolaris/uts=
/common/fs/zfs/arc.c

A few threads on zfs-dicuss on this:

=
http://mail.opensolaris.org/pipermail/zfs-discuss/2011-April/thread.html#4=
8026
=
http://mail.opensolaris.org/pipermail/zfs-discuss/2011-May/thread.html#481=
85

Also, the above numbers are for OpenSolaris: someone may want to check =
the structure sizes for FreeBSD to be sure. They should get you in the =
right ballpark though.