zfs: is deduplication retained across zfs send/zfs receive?

Moritz Wilhelmy

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Jun 4, 2013, 10:39:13 AM6/4/13

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Hello,

Does anybody know whether a "zfs send" image of a deduplicated zfs
volume stays deduplicated after I "zfs receive" it on another machine?
This would be useful, because the other machine (which is intended for
backups) is too slow and doesn't have enough RAM to take care of
deduplication (besides, deduplication is already done on the sending
machine, so theoretically it would be stupid to do it twice, if the
architecture of ZFS permits this).

Best regards,

Moritz

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Ian Collins

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Jun 4, 2013, 4:22:17 PM6/4/13

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Moritz Wilhelmy wrote:
> Hello,
>
> Does anybody know whether a "zfs send" image of a deduplicated zfs
> volume stays deduplicated after I "zfs receive" it on another machine?

Only if dedup is enabled on the other machine and it may not have the
same degree of deduplication.

> This would be useful, because the other machine (which is intended for
> backups) is too slow and doesn't have enough RAM to take care of
> deduplication (besides, deduplication is already done on the sending
> machine, so theoretically it would be stupid to do it twice, if the
> architecture of ZFS permits this).

I don't think it does, or can. Consider the case of sending some, but
not all, filesystems from a pool. Dedup is applied (although not
enabled) to the pool as a whole, not per filesystem.

--
Ian Collins

cindy swearingen

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Jun 5, 2013, 6:31:36 PM6/5/13

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If I'm remembering correctly, zfs send does not send a dedup'ed stream
unless
the -D option is used even if the file system has dedup enabled so the
original
use case should be okay.

I can confirm back at the office tomorrow.

Thanks, Cindy

Ian Collins

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Jun 5, 2013, 6:50:57 PM6/5/13

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cindy swearingen wrote:
> On Jun 4, 2:22 pm, Ian Collins <ian-n...@hotmail.com> wrote:
>> Moritz Wilhelmy wrote:
>>> Hello,
>>
>>> Does anybody know whether a "zfs send" image of a deduplicated zfs
>>> volume stays deduplicated after I "zfs receive" it on another machine?
>>
>> Only if dedup is enabled on the other machine and it may not have the
>> same degree of deduplication.
>>
>>> This would be useful, because the other machine (which is intended for
>>> backups) is too slow and doesn't have enough RAM to take care of
>>> deduplication (besides, deduplication is already done on the sending
>>> machine, so theoretically it would be stupid to do it twice, if the
>>> architecture of ZFS permits this).
>>
>> I don't think it does, or can. Consider the case of sending some, but
>> not all, filesystems from a pool. Dedup is applied (although not
>> enabled) to the pool as a whole, not per filesystem.
>

> If I'm remembering correctly, zfs send does not send a dedup'ed stream
> unless
> the -D option is used even if the file system has dedup enabled so the
> original
> use case should be okay.

You are remembering correctly. Not all receivers can handle a
deduplicated stream.

--
Ian Collins

cindy swearingen

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Jun 6, 2013, 11:00:41 AM6/6/13

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Yes, thanks. In addition, the zfs send -D acts somewhat like a toggle.
If the file system has dedup enabled, you need to use -D to send dedup
data.
If the file system has dedup disabled, you can use -D to send dedup'ed
data.

Thanks, Cindy

Moritz Wilhelmy

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Jun 10, 2013, 7:44:49 AM6/10/13

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Hello,

cindy swearingen <cindy.sw...@gmail.com> wrote:
> Yes, thanks. In addition, the zfs send -D acts somewhat like a toggle.
> If the file system has dedup enabled, you need to use -D to send dedup
> data.
> If the file system has dedup disabled, you can use -D to send dedup'ed
> data.

Thanks for the informative reply, I'll try this.

Best,