Disk Partiy and staggered drive spin up...

[thestraycat]

Hi guys,

 

A nice simple and easy one here. Assuming all your drives are spun down, and the scheduler decies its time for a parity check... Does unraid stagger the disks up in batches. Or does it issue the command all at once??

 

The reason i ask is that i have 24 drives and speculate that i have PSU issues. I've discussed it in more detail over on this thread: 

 

however, it would make sense if unraid were to do a pre-check on prior to starting a parity sync by issueing each drive to spin up individually a second or so apart waiting for a minute or so for all drives to be up, and then starting the parity check when all drives have spun up to avoid an insurge of current for disks that were previously spun down to come up at once. I assumed this may actually be already the case, but all the threads that ive found mention that unraid dosnt have the potential to stagger spinning drives up so all drives will come up at once at the beginning of a parity check.

 

Can someone clear this up for me?

 

[trurl]

Parity check isn't the only time all drives need to spin up. Emulating a disabled disk, for example, requires all disks to spin. Could cause quite a delay in file access to stagger them. Suggest you fix your power issue.

[JonathanM]

If your power supply can't handle spinning up all array drives simultaneously your only recourse is to disable spin down.

 

Consider this scenario, which is rather common.

 

A read fails on a drive. Unraid immediately spins up all drives, calculates what should have been returned from the failed read, and writes it to the drive as well as returning the data successfully to the calling process. If the write succeeds, the error counter for the drive that failed the read is incremented, and everything moves on as normal. If the write fails, the drive is red balled and all further writes to that data slot are emulated with the rest of the drives and parity.

 

That is exactly the worst possible time for a power supply to sag, as it will likely cause multiple drives to fail their read, causing chaos, very likely corrupting data.

 

Either be sure your PSU can handle all parity protected drives spinning up simultaneously, or disable spin down. No other RAID solution allows pool drives to spin down individually, so it's just the price you pay to use Unraid's extra capabilities.