Hi,

I’m planning to migrate my NAS from Synology to Unraid. My current drives are:

• 4 × 22TB SATA drives + 1 × 18TB SATA drive (data disks)

• 2 × 22TB SAS drives (intended as parity disks)

I have a few questions:

1. Can SAS drives be used as parity for SATA data drives in Unraid?

2. Even though all drives are nominally 22TB, could there be slight differences in usable sector count? If a SAS drive has fewer usable sectors than the 22TB SATA drives, would it be rejected as a parity disk? In other words, does Unraid require the parity drive to have the largest sector count, even if the data disks are far from full?

3. Would using a larger parity drive, e.g., 24TB, be safer to avoid issues?

Thanks for any advice!

SAS is just a different interface, the platters and formatting themselves are usually identical between the disks. IE, a WD HC530 14TB SAS disk is 100% identical to a WD HC530 14TB SATA disk, other than the physical interface and controller on the PCB of the drive. So yes, it's absolutely possible and logical to use a SAS disk for parity. You gain nothing with SAS over SATA. You might encounter spin-down issues with SAS disks, which will cost you more power. The HBA will certainly cost you more power as well as most of them don't support ASPM unless you've bough a recent generation HBA.

2) Plausible, but unlikely.

3) That would certainly eliminate any potential issues, but those issues are so slim that it's unlikely to occur in the first place.

22 minutes ago, xemaco said:

Even though all drives are nominally 22TB, could there be slight differences in usable sector count? If a SAS drive has fewer usable sectors than the 22TB SATA drives, would it be rejected as a parity disk? In other words, does Unraid require the parity drive to have the largest sector count, even if the data disks are far from full?

Yes, UNRAID is very picky about this. It needs the parity drive to map EVERY sector of the largest data drive.

The SAS HBA could use a slightly different partitioning scheme, it may "steal" some sectors (or tracks) for its own management infos to store. These sectors are not visible to the OS below but obviously lower to the total available sector count. Which HBA needs it and which one not, is not visible beforehand, you have to try it out. But it CAN happen that the same type of drive shows different sector count for SATA and SAS Drives. (The same is true for many external USB enclosures. The drives formatted in them show different layout and freespace compared to "direct connected" to the computer. The data can only be used while the drive is in the enclosure)

But of course, you can then simply switch around and use a SATA disk as parity and the SAS drive for data. As long, there is no real data on the drives, it does not matter.

Edited September 30, 2025Sep 30 by MAM59

8 hours ago, Brandon_K said:

SAS is just a different interface, the platters and formatting themselves are usually identical between the disks. IE, a WD HC530 14TB SAS disk is 100% identical to a WD HC530 14TB SATA disk, other than the physical interface and controller on the PCB of the drive. So yes, it's absolutely possible and logical to use a SAS disk for parity. You gain nothing with SAS over SATA. You might encounter spin-down issues with SAS disks, which will cost you more power. The HBA will certainly cost you more power as well as most of them don't support ASPM unless you've bough a recent generation HBA.

2) Plausible, but unlikely.

3) That would certainly eliminate any potential issues, but those issues are so slim that it's unlikely to occur in the first place.

thank you very much

8 hours ago, MAM59 said:

Yes, UNRAID is very picky about this. It needs the parity drive to map EVERY sector of the largest data drive.

The SAS HBA could use a slightly different partitioning scheme, it may "steal" some sectors (or tracks) for its own management infos to store. These sectors are not visible to the OS below but obviously lower to the total available sector count. Which HBA needs it and which one not, is not visible beforehand, you have to try it out. But it CAN happen that the same type of drive shows different sector count for SATA and SAS Drives. (The same is true for many external USB enclosures. The drives formatted in them show different layout and freespace compared to "direct connected" to the computer. The data can only be used while the drive is in the enclosure)

But of course, you can then simply switch around and use a SATA disk as parity and the SAS drive for data. As long, there is no real data on the drives, it does not matter.

thank you very much. so clear and kind