Can partitions of 2 mirrored ext4 boot disks be used as a zfs special device?

[atatury]

Hi all,

Let's say we have a backup server having 3 500GB ssd disks and 7 6TB hdd disks. We want to make a PBS installation to one of the ssd disks and boot device should be a 100 GB ext4 partition. Then boot device partition is triple mirrored to other 2 ssd disk partitions via linux LVM and we want to make use the rest of the free space (400GB) as a zfs special device for the upcoming raidz2 zfs pool of hdds. Is this a perfectly legal configuration ? It's not absolutely clear from documentation.

Thank you..

 

[UdoB]

The pure fact that you ask this tells me that you should NOT do that.

Technically anything goes.

Using only your available disks I would do it this way: install PBS on all (and only) the harddisks, in pairs or course. You have 7 and I would only use 6 for three mirrors. This results in something similar a Raid10. Each one of these six disks is bootable at the end - take this as a bonus goodie.

After initial setup add a "Special Device". As far as I know this must be done via the CLI. If those SSDs are "Enterprise class with PLP" two of them are enough. If they are cheaper opt for a single tripple mirror. Yes, all three will have the very same content and that's a well supported construct.

Search for and read about "small_blocks" in this context. They are helpful to optimize the usage of the "too large for only meta-data" SSD.

Just my 2 €¢...

 

[atatury]

The pure fact that you ask this tells me that you should NOT do that.

Technically anything goes.

Using only your available disks I would do it this way: install PBS on all (and only) the harddisks, in pairs or course. You have 7 and I would only use 6 for three mirrors. This results in something similar a Raid10. Each one of these six disks is bootable at the end - take this as a bonus goodie.

After initial setup add a "Special Device". As far as I know this must be done via the CLI. If those SSDs are "Enterprise class with PLP" two of them are enough. If they are cheaper opt for a single tripple mirror. Yes, all three will have the very same content and that's a well supported construct.

Search for and read about "small_blocks" in this context. They are helpful to optimize the usage of the "too large for only meta-data" SSD.

Just my 2 €¢...

Thanks for your input. But in your case, zfs pool doesn't meet the size requirements, for my case pool size shouldn't be less than 20TB and raid10 size is about 3x6=18 TB whereas raidz2 size is about 6x4=24TB. Sorry I forgot to mention about size requirement.

Maybe I should add 2 more hdds for boot disks. Does this sound good ?

 

[UdoB]

Maybe I should add 2 more hdds for boot disks. Does this sound good ?

Sure!

(( Note that in my example there is no specific boot disk - all of the hard disk are bootable. ))

More vdevs do increase both capacity and IOPS. Especially for rotating rust: you just can't have enough of them ;-)

The only counter arguments are well known: you need the space/connectors for them, they will cost money and they produce heat.