SAS vs SATA and Recovery

SAS and SATA are electrically compatible to a degree that allows connecting a SATA storage device to a SAS controller. The SAS controller understands the SATA protocol so this works. A SAS device can’t be physically connected to a SATA controller and if you did manage to connect it then it wouldn’t work.

Some SAS RAID controllers don’t permit mixing SAS and SATA devices in the same array, this is a software issue and could be changed. I know that the PERC controllers used by Dell (at least the older versions) do this and it might affect many/most MegaRAID controllers (which is what PERC is).

If you have a hardware RAID array of SAS disks and one fails then you need a spare SAS disk and as the local computer store won’t have any you need some on hand.

The Linux kernel has support for the MegaRAID/PERC superblocks so for at least some of the RAID types supported by MegaRAID/PERC you can just connect the disks to a Linux system and have it work (I’ve only tested on JBOD AKA a single-disk RAID-0). So if you have a server from Dell or IBM or any other company that uses MegaRAID which fails you can probably just put the disks into a non-RAID SAS controller and have them work. As Linux doesn’t care about the difference between SAS and SATA at the RAID level you could then add a SATA disk to an array of SAS disks. If you want to move an array from a dead Dell to a working IBM server or the other way around then you need it to be all SATA or all SAS. You can use a Linux system to mount an array used by Windows or any other OS and then migrate the data to a different array.

If you have an old array of SAS disks and one fails then it might be a reasonable option to just migrate the data to a new array of SATA SSDs. EG if you had 6*600G SAS disks you could move to 2*4TB SATA SSDs and get more storage, much higher performance, less power use, and less noise for a cost of $800 or so (you can spend more to get better performance) and some migration time.

Having a spare SAS controller for data recovery is convenient. Having a spare SAS disk for any RAID-5/RAID-6 is a good thing. Having lots of spare SAS disks probably isn’t useful as migrating to SATA is a better choice. SATA SSDs are bigger and faster than most SAS disks that are in production. I’m sure that someone uses SAS SSDs but I haven’t yet seen them in production, if you have a SAS system and need the performance that SSDs can give then a new server with U.2 (the SAS equivalent of NVMe) is the way to go). SATA hard drives are also the solution for seriously large storage, 16TB SATA hard drives are cheap and work in all the 3.5″ SAS systems.

It’s hard to sell old SAS disks as there isn’t much use for them.

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