questions about migration from ZEVO

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questions about migration from ZEVO

Postby antikus » Thu Dec 17, 2015 2:53 pm

I have two pools created by ZEVO. One serves as primary storage and the other as an offsite backup where I send/recv data every night. I'm looking to move from ZEVO to O3X. Questions (I'm sorry if that has been covered before, but I did search the forum before posting. Honest.):

1. If I replace ZEVO with O3X on one machine only, will the send/recv still work between the machines?

2. Do I need to rebuild my pool to get access to ZVOL features? Or, can I just upgrade the existing pool without moving the data around?

3. Would there be any performance (speed) benefits if I rebuild the pool from scratch. (I vaguely recall someone recommending to rebuild pools when moving to O3X, but that was at least 2 years ago.)

4. A very noob question: if I allocate a 1tb ZVOL on a 5 tb pool, does it mean that the available pool space is now 4tb even if the zvol is empty? This in contrast to zfs filesystem, which shares the available space among all filesystems on the pool (unless otherwise specified, of course)

Thank you for your help!

-- Anton
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Re: questions about migration from ZEVO

Postby lundman » Thu Dec 17, 2015 5:31 pm

antikus wrote:I have two pools created by ZEVO. One serves as primary storage and the other as an offsite backup where I send/recv data every night. I'm looking to move from ZEVO to O3X. Questions (I'm sorry if that has been covered before, but I did search the forum before posting. Honest.):

1. If I replace ZEVO with O3X on one machine only, will the send/recv still work between the machines?


If you do not upgrade the pool version, then you can still use send/recv. You can upgrade the receiving pool too. But if the sender is newer than receiver it will not accept the stream. It is more about the pool version, and dataset version, and not so much about the implementation of ZFS (beyond what pool and dataset versions it supports)

2. Do I need to rebuild my pool to get access to ZVOL features? Or, can I just upgrade the existing pool without moving the data around?


You can use ZVOLs without upgrading AFAIK. You can also upgrade the pool version, and dataset version without moving data around.


3. Would there be any performance (speed) benefits if I rebuild the pool from scratch. (I vaguely recall someone recommending to rebuild pools when moving to O3X, but that was at least 2 years ago.)


This depends, if you use compression=lz4, and you probably want to as it is considerably better, you are better off starting from scratch, since setting compression= only affects things written to the pool after the setting. So upgrading your pool, and then setting compression=lz4 will not get you the benefits of it. (If lz4 is enabled, it will also use it on the metadata, not just your file data).
So in a way, I would recommend starting from scratch, BUT, you do not have to.. It will work fine keeping the version as old version (but without all the new features) and it will work fine if you upgrade (getting you near features).

4. A very noob question: if I allocate a 1tb ZVOL on a 5 tb pool, does it mean that the available pool space is now 4tb even if the zvol is empty? This in contrast to zfs filesystem, which shares the available space among all filesystems on the pool (unless otherwise specified, of course)


This is actually controlled with refreservation setting. By default, the ZVOL will not "pre-assign" the space needed for the 1tb. So you are still sharing space with the pool, and could potentially run out of space (in the pool) before the theoretical 1Tb ZVOL is full. Now if you DO want the ZVOL to take the space up in advance, even if empty, you set zfs set refreservation=1TB POOL/zvol. This way you know your 1TB ZVOL will never (falsely) be out of space. It is a good idea to set refreservation on swap ZVOLs for obvious reasons.
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