Different redundancy within the pool. Win8 can, ZFS cannot?

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Different redundancy within the pool. Win8 can, ZFS cannot?

Post by si-ghan-bi » Mon Oct 29, 2012 3:17 am

http://arstechnica.com/information-tech ... n-it-works

Windows 8 Storage Spaces allows the user to define some data to be stored with high redundancy (2-way, 3-way) and some data with less redundancy (RAID5 equivalent). Is this somehow possible with ZFS? it's a killer feature for home users, since most of the data don't require mirror-safety (films, music) and some other would benefit from even a 3-way mirror. At least, I would like it a lot.

Is this possible by using different datasets within the same pool? I don't think so, but I ask :)
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Re: Different redundancy within the pool. Win8 can, ZFS cann

Post by jollyjinx » Mon Oct 29, 2012 9:27 am

you can. It's called ditto blocks.

Create a filesystem e.g. tank/yourfs and then set the number of copies onto it:

zfs set copies=3 tank/yourfs
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Re: Different redundancy within the pool. Win8 can, ZFS cann

Post by si-ghan-bi » Mon Oct 29, 2012 11:28 am

I thought it applied to whole pools, not to dataset. Thank you.

However, that allows me to store multiple copies, but the underlying pool has to be for example RAIDZ for this to work: a dataset with copies=1 is a standard RAIDZ dataset, copies=2..n_disks to have higher redundancy. However, what will happen if two disks of the 3-way RAIDZ pool are lost? Will ZFS read anyway the files copied with copies=3 ?
Unfortunately I don't have spare disks to test. I can try with a VM, for example nas4free.
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Re: Different redundancy within the pool. Win8 can, ZFS cann

Post by shuman » Mon Oct 29, 2012 1:34 pm

From: https://blogs.oracle.com/relling/entry/zfs_copies_and_data_protection

Under the Covers

ZFS will spread the ditto blocks across the vdev or vdevs to provide spatial diversity. Bill Moore has previously blogged about this, or you can see it in the code for yourself. From a RAS perspective, this is a good thing. We want to reduce the possibility that a single failure, such as a drive head impact with media, could disturb both copies of our data. If we have multiple disks, ZFS will try to spread the copies across multiple disks. This is different than mirroring, in subtle ways. The actual placement is ultimately based upon available space. Let's look at some simplified examples. First, for the default file system configuration settings on a single disk. . .
- Mac Mini (Late 2012), 10.8.5, 16GB memory, pool - 2 Mirrored 3TB USB 3.0 External Drives
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cross reference

Post by grahamperrin » Mon Oct 29, 2012 2:02 pm

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Re: Different redundancy within the pool. Win8 can, ZFS cann

Post by si-ghan-bi » Tue Oct 30, 2012 6:09 am

Sorry, but I still don't understand: if I have a RAIDZ with 3 disks where a specific dataset is using copies=3 (therefore one whole copy per disk) and I lose 2 disks, thus making the RAIDZ unrecoverable, will I still be able to retrieve the files of that specific dataset? I think I will have to try because the articles doesn't clarify this case.
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